# coding:utf-8

D = int(input())
C = input()
C = C + input()
ans = [0]
for i in range(14):
    for j in range(i, 15):
        e = C[i:j]
        for k in range(len(e)):
            if e.count('x') <= D:
                ans.append(len(e))

print(max(ans))