n = int(input())
t = input()+input()+"x"
ans = 0
for i in range(15-n):
    s = t[:i] + "o"*n + t[i+n:]
    v = 0
    for i in s:
        if i == "o":
            v += 1
        else:
            ans = max(ans,v)
            v = 0
print(ans)