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