#!/usr/bin/env python3
d = int(input())
c = list(('x' * 14) + input() + input() + ('x' * 14))
ans = 0
for i in range(42):
    t = c[:]
    for j in range(d):
        if i + j >= 42 or t[i + j] == 'o':
            break
        t[i + j] = 'o'
    t = ''.join(t)
    ans = max(ans, max([len(x) for x in t.split('x')]))
print(ans)