不知道哪里错了，求大佬帮我改一下啊啊啊

#include<iostream>
#include<string>

using namespace std; 

int main()
{
    long long N, LL;
    int L, M;
    string str;
    
    cin >> str;
    cin >> N;
    
    L = str.length();
    LL = L;
    M = 0;
    for(M = 0; M < 64 && LL < N; M++)
    {
    	LL *= 2;
	}
	
	for(; M > 0; M--)
    {
    	if(N > LL/2)
    	{
    		N = N - LL/2 - 1;
		}
    	LL = LL/2;
	}
	
    cout << str[N-1];

    return 0;
}