d = int(input())
s = 'x' * d
for _ in range(2):
    s += input()
s += 'x' * d

ans = 0
n = len(s)
for i in range(n):
    j = i
    x = 0
    while j < n:
        if s[j] == 'x':
            x += 1
            if x == d + 1:
                break
        j += 1
    ans = max(ans, j - i)
print(ans)