#!/usr/bin/env python3
def takewhile(s, c):
    assert c in 'ox'
    f = {'o':'x', 'x':'o'}
    try:
        return s[:s.index(f[c])]
    except:
        return s
d = int(input())
s = ''
s += input()
s += input()
ans = 0
for i in range(len(s)):
    x = len(takewhile(s[i:], 'o'))
    y = min(d, len(takewhile(s[i+x:], 'x')))
    z = len(takewhile(s[i+x+y:], 'o'))
    ans = max(ans, x + y + z)
print(ans)