insts = list(map(int, open(0).read().strip().splitlines()))
part_2 = True
c = 0
i = 0
while i < len(insts):
    ci = i
    i += insts[i]
    if part_2:
        if insts[ci] >= 3:
            insts[ci] -= 1
        else:
            insts[ci] += 1
    else:
        insts[ci] += 1
    c += 1
print(c)