d = int(input())
s = input() + input()
ans = 0
def g(z):
    cnt = 0
    num = 0
    for a in z + 'x':
        if a == 'o':
            num += 1
        else:
            cnt = max(cnt, num)
            num = 0
    global ans
    ans = max(ans, cnt)
def f(a, s, b):
    if b == 0:
        g(s)
        return
    for i in range(a, 14):
        if s[i] == 'x':
            f(i + 1, s[:], b)
            f(i + 1, s[:i] + 'o' + s[i + 1:], b - 1)
f(0, s, d)
print(ans)