# coding:utf-8

D = int(input())
C = 'xxxxxxxxxxxxxxx' + input()
C = C + input() + 'xxxxxxxxxxxxxxxxx'
ans = [0]
for i in range(len(C)+1):
    for j in range(i, len(C)+1):
        e = C[i:j]
        for k in range(len(e)):
            if e.count('x') <= D:
                ans.append(len(e))

print(max(ans))