D = int(input())
# xが平日, oが休日
s = "x"*D
s += str(input())
s += str(input())
s += "x"*D
ans = 0

# xをoにしたら飛ばすようにする
for i in range(len(s)):
    cnt = i
    tmp = 0
    hoge = D
    while len(s) > cnt:
        if s[cnt] == "x":
            if hoge <= 0:
                break
            else:
                for _ in range(hoge):
                    if cnt >= len(s):
                        break
                    if s[cnt] == "o":
                        cnt += 1
                        tmp += 1
                        break
                    cnt += 1
                    tmp += 1
                    hoge -= 1
                hoge = 0
        elif s[cnt] == "o":
            tmp += 1
            cnt += 1
    ans = max(ans, tmp)
    
print(ans)