d = int(input())
s = input() + input()
r = p = 0
for i in range(len(s)):
    if s[i] == 'x':
        p = 0
    v = p
    w = d
    for j in range(i, len(s)):
        if s[j] == 'x':
            if w == 0:
                break
            w -= 1
        elif w != d:
            w = 0
        v += 1
    r = max(r, v)
    if s[i] == 'o':
        p += 1
print(r)