代码和题解类似，但是为什么超时了？

#include<iostream>
#include<string>
using namespace std;
long long n, len, i;
string s;
int main() {
    cin >> s >> n;
    len = s.length();
    while(n > s.length()) {
        while (len < n) len *= 2;
        len = len / 2;
        n -= 1 + len;
        if (n == 0) n = len;

    }
    cout << s[n - 1] << endl;
    return 0;
}