diff --git a/README.md b/README.md
index 948a238..1246e53 100644
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 
 - Day 1: 40:00 (I don't know what I am doing.)
 - Day 2: 14:15 (Okay, but far way from leaderboard.)
-- Day 3: 1st 20:00, 2nd 70:00... (I had a logic error that took me a while to find.) 
+- Day 3: 1st 20:00, 2nd 70:00... (I had a logic error that took me a while to find.)
 - Day 4: 1st 9:06, 2nd 22:31; it wasn't hard but I didn't think quick enough :/
 - Day 5: 1st 25:00, 2nd 1:55:00; Required patience and accuracy
 - Day 6: 13:54; I was slow because I thought it is much harder?
@@ -12,7 +12,9 @@
 - Day 10: 180:00; this one was hard for me.
 - Day 11: 68:00; okay but not elegant and way too slow ofc; x-ray solution would have been neat
 - Day 12: 52:00 and 22:00 for leaderboard; had the right idea and I am good at this type of problem
-- ...
+- Day 13: 90:00; pretty straightforward  but way too slow
+- Day 14:
+- Day 15:
 - Day 16: 00:27:30 745; best placement so far, of course still horribly slow
 - Day 17: a couple of hours; I realized that I need A* after a while; reused
   implementation from Project Euler but improved with heapq which was super fun
diff --git a/d13.py b/d13.py
new file mode 100644
index 0000000..1c80e25
--- /dev/null
+++ b/d13.py
@@ -0,0 +1,92 @@
+from lib import *
+
+EXAMPLE = """
+#.##..##.
+..#.##.#.
+##......#
+##......#
+..#.##.#.
+..##..##.
+#.#.##.#.
+
+#...##..#
+#....#..#
+..##..###
+#####.##.
+#####.##.
+..##..###
+#....#..#
+""".replace("\n", "", count=1)
+
+
+def solve(i: Input, second=False):
+    res = 0
+    ps = i.paras()
+
+    for p in ps:
+        g = Input(p).grid2()
+
+        row_res, col_res = [], []
+        rows = g.rows()
+        cols = g.cols()
+        for i_row in range(len(rows) - 1):
+            for i in range(min(i_row + 1, len(rows) - i_row - 1)):
+                if rows[i_row - i] != rows[i_row + i + 1]:
+                    break
+            else:
+                if not second:
+                    res += (i_row + 1) * 100
+                row_res.append(i_row)
+
+        for i_col in range(len(cols) - 1):
+            for i in range(min(i_col + 1, len(cols) - i_col - 1)):
+                if cols[i_col - i] != cols[i_col + i + 1]:
+                    break
+            else:
+                if not second:
+                    res += (i_col + 1)
+                col_res.append(i_col)
+
+        if not second:
+            continue
+
+        for c in g.all_coords():
+            g[c] = '.' if g[c] == '#' else '#'
+            rows = g.rows()
+            cols = g.cols()
+            for i_row in range(len(rows) - 1):
+                for i in range(min(i_row + 1, len(rows) - i_row - 1)):
+                    if rows[i_row - i] != rows[i_row + i + 1]:
+                        break
+                else:
+                    if not i_row in row_res:
+                        res += (i_row + 1) * 100
+                        row_res.append(i_row)
+
+            for i_col in range(len(cols) - 1):
+                for i in range(min(i_col + 1, len(cols) - i_col - 1)):
+                    if cols[i_col - i] != cols[i_col + i + 1]:
+                        break
+                else:
+                    if not i_col in col_res:
+                        res += (i_col + 1)
+                        col_res.append(i_col)
+            g[c] = '.' if g[c] == '#' else '#'
+        if len(row_res + col_res) != 2:
+            raise Exception("Unexpected amount of mirrors!!")
+    return res
+
+
+def main():
+    DAY_INPUT = "i13.txt"
+
+    print("Example  1:", solve(Input(EXAMPLE)))
+    print("Solution 1:", solve(Input(DAY_INPUT)))
+    assert 29846 == solve(Input(DAY_INPUT))
+
+    print("Example  2:", solve(Input(EXAMPLE), True))
+    print("Solution 2:", solve(Input(DAY_INPUT), True))
+    assert 25401 == solve(Input(DAY_INPUT), True)
+
+if __name__ == "__main__":
+    main()
diff --git a/d18.py b/d18.py
index cf8b52c..26d4e63 100644
--- a/d18.py
+++ b/d18.py
@@ -51,8 +51,7 @@ def solve(i: Input, second=False):
                 to_visit.append(nc)
     return len(set(coords))
 
-
-def calc(ins):
+def area(ins):
     c = (0, 0)
     corners = []
     for i, (count, d) in enumerate(ins):
@@ -86,6 +85,20 @@ def calc(ins):
     return int(shoelace_area(corners))
 
 
+def area2(ins):
+    # Solution based on Shoelace area and the idea that the outside area is
+    # perimate * 1 // 2, and then +1 (for four quarter corners). All other
+    # corners cancel out to zero.
+    c = (0, 0)
+    corners = []
+    perimeter = 0
+    for i, (count, d) in enumerate(ins):
+        c = (c[0] + count * d[0], c[1] + count * d[1])
+        corners.append(c)
+        perimeter += count
+    return int(shoelace_area(corners)) + perimeter // 2 + 1
+
+
 def solve2(i: Input, second=False):
     lines = i.lines()
     ins = []
@@ -97,7 +110,8 @@ def solve2(i: Input, second=False):
         d = int(c[:5], 16)
         m = {"0": Grid2D.E, "1": Grid2D.S, "2": Grid2D.W, "3": Grid2D.N}[c[5]]
         ins.append((d, m))
-    return calc(ins)
+    assert area(ins) == area2(ins)
+    return area(ins)
 
 
 def debug():
@@ -113,7 +127,7 @@ def debug():
     ]
     # Should be 14 but is 2 because it's going counter-clockwise, but mapping
     # only works for clockwise.
-    print(calc(ins))
+    print(area(ins))
 
 
 def main():
diff --git a/lib.py b/lib.py
index 19644de..1a1abc3 100644
--- a/lib.py
+++ b/lib.py
@@ -133,7 +133,7 @@ class Input:
         return self.text.splitlines()
 
     def paras(self) -> list[list[str]]:
-        return [p.splitlines() for p in self.text.split("\n\n")]
+        return [p for p in self.text.split("\n\n")]
 
     def grid2(self) -> Grid2D:
         return Grid2D(self.text)