diff --git a/.gitignore b/.gitignore
index 5da92f7..6f6f9cd 100644
--- a/.gitignore
+++ b/.gitignore
@@ -13,3 +13,6 @@ Cargo.lock
 # Sublime
 algos.sublime-project
 algos.sublime-workspace
+
+# Data for the exercises. Some files are big so don't check them in.
+data/c2a4.txt
diff --git a/misc/birthday_problem.py b/misc/birthday_problem.py
new file mode 100644
index 0000000..174f8b7
--- /dev/null
+++ b/misc/birthday_problem.py
@@ -0,0 +1,16 @@
+def p_all_different(n):
+    p = 1
+    free_days = 365
+    days_year = 365
+    for i in range(0, n):
+        p = p * (free_days / days_year)
+        free_days -= 1
+    return p
+
+
+if __name__ == "__main__":
+    for i in range(1, 30):
+        print(f"p_all_different({i})={p_all_different(i)}")
+        if  p_all_different(i) < 0.5:
+            break
+
diff --git a/png/dijkstra_1.png b/misc/dijkstra_1.png
similarity index 100%
rename from png/dijkstra_1.png
rename to misc/dijkstra_1.png
diff --git a/png/dijkstra_2.png b/misc/dijkstra_2.png
similarity index 100%
rename from png/dijkstra_2.png
rename to misc/dijkstra_2.png
diff --git a/png/dijkstra_3.png b/misc/dijkstra_3.png
similarity index 100%
rename from png/dijkstra_3.png
rename to misc/dijkstra_3.png
diff --git a/src/merge_sort.py b/misc/merge_sort.py
similarity index 100%
rename from src/merge_sort.py
rename to misc/merge_sort.py
diff --git a/src/heap.rs b/src/heap.rs
index 7251e6c..005ca85 100644
--- a/src/heap.rs
+++ b/src/heap.rs
@@ -91,9 +91,9 @@ impl<T: std::cmp::PartialOrd> MinHeap<T> {
     }
 }
 
-pub fn heap(v: &Vec<i32>) -> i32 {
-    let mut hl: MinHeap<i32> = MinHeap::new();
-    let mut hh: MinHeap<i32> = MinHeap::new();
+pub fn heap(v: &Vec<i64>) -> i64 {
+    let mut hl: MinHeap<i64> = MinHeap::new();
+    let mut hh: MinHeap<i64> = MinHeap::new();
 
 	let mut iter = v.into_iter();
     let mut current_median = *iter.next().unwrap();
diff --git a/src/main.rs b/src/main.rs
index 40d10f6..a04d0d1 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -5,15 +5,16 @@ mod min_cut;
 mod quick_sort;
 mod ssc;
 mod util;
+mod two_sum;
 
 use std::cmp::min;
-
 use crate::dijkstra::dijkstra;
 use crate::heap::heap;
 use crate::merge_sort::merge_sort_inversions;
 use crate::min_cut::min_cut;
 use crate::quick_sort::quick_sort;
 use crate::ssc::ssc;
+use crate::two_sum::find_two_sums;
 
 #[allow(dead_code)]
 fn c1a2() {
@@ -70,16 +71,24 @@ fn c2a2() {
 
 #[allow(dead_code)]
 fn c2a3() {
-    let vec = util::read_to_vector("data/c2a3.txt").unwrap();
-    let r = heap(&vec);
+    let v = util::read_to_vector("data/c2a3.txt").unwrap();
+    let r = heap(&v);
     println!("c2a3={:?}", r);
 }
 
+#[allow(dead_code)]
+fn c2a4() {
+    let v = util::read_to_vector("data/c2a4.txt").unwrap();
+    let r = find_two_sums(v);
+    println!("c2a4={:?}", r);
+}
+
 fn main() {
     // c1a2();
     // c1a3();
     // c1a4();
     // c2a1();
     // c2a2();
-    c2a3();
+    // c2a3();
+    c2a4();
 }
diff --git a/src/two_sum.rs b/src/two_sum.rs
new file mode 100644
index 0000000..cd38aa0
--- /dev/null
+++ b/src/two_sum.rs
@@ -0,0 +1,13 @@
+
+fn two_sum(t: i64, xs_vec: &Vec<i64>) {
+
+}
+
+pub fn find_two_sums(xs: Vec<i64>) -> usize {
+
+
+
+	0
+}
+
+
diff --git a/src/util.rs b/src/util.rs
index eab8a95..1073b74 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -23,7 +23,7 @@ pub struct DirectedWeightedGraph {
     pub distance: Vec<usize>,
 }
 
-pub fn read_to_vector(path: &str) -> Result<Vec<i32>, io::Error> {
+pub fn read_to_vector(path: &str) -> Result<Vec<i64>, io::Error> {
     let file = File::open(path)?;
     let br = BufReader::new(file);
     let mut v = Vec::new();