#!/usr/bin/env python
# -*- coding: utf-8 -*-

d = int(input())
s = "xxxxxxxxxxxxxx" + input() + input() + "xxxxxxxxxxxxxx"

ans = 0
for i in range(0, 29):
    check = list(s);
    for j in range(i, i + d):
        if check[j] == "o": break;
        check[j] = "o"
    cum = [0] * (len(s) + 1)
    for j in range(0, len(s)):
        if check[j] == "o": cum[j] = 1;
    for j in range(1, len(s)):
        if cum[j] == 0: continue;
        cum[j] += cum[j - 1];
        ans = max(ans, cum[j])
print(ans)