#!/usr/bin/env python3
d = int(input())
c = input() + input()
c = 'x' * 20 + c + 'x' * 20
c = list(c)
ans = 0
for i in range(35):
    t = c[:]
    for j in range(d):
        t[i + j] = 'o'
    t = ''.join(t)
    ans = max(ans, max([len(x) for x in t.split('x')]))
print(ans)