D = int(raw_input())
s = list(raw_input() + raw_input())
ans = 0
for i in xrange(14 - D):
    ss = s[:]
    ss[i:i + D] = "o" * D
    cnt = 0
    for si in ss:
        if si == "o": cnt += 1
        else:
            ans = max(ans, cnt)
            cnt = 0
    ans = max(ans, cnt)
print ans