90分求助

第二个测试点没过...

#include<iostream>
#include<cstring>
#include<cmath>
using namespace std;
int main()
{
    string a;
    long long n, t = 0;
    cin >> a;
    cin >> n;
    long long len = a.size(), cnt = log10((n - 1) / len) / log10(2);
    while (n > len)
    {
        long long temp = n;
        n = n - pow(2, cnt) * len - 1;
        if (n <= 0)n = temp - 1;
        cnt = log10((n - 1) / len) / log10(2);
    }
    cout << a[n - 1];
}

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