from sys import stdin, stdout

input = lambda: stdin.readline()
write = stdout.write

def longest(seq):
    max_count = 0
    prev_char = None
    for current in seq:
        if current == 'x':
            prev_char = 'x'
            continue
        if prev_char != current:
            count = 1
        else:
            count += 1
        if count > max_count:
            max_count = count
        prev_char = current
    return max_count

def main():
    D = int(input())
    C = stdin.read().splitlines()

    c = list(C[0]) + list(C[1])
    gw = longest(c)
    if not D:
        print(gw)
        return
    for i in range(15 - D):
        c_ = c[:]
        if not 'o' in c_[i:i+D]:
            c_[i:i+D] = ['o'] * D
            gw = max(gw, longest(c_))
    print(gw)

main()