How do I get my code to work for large values of N?

Question

The input consists of a string followed by N. The string consists of at most 30 uppercase characters, and N≤10¹⁸. The code outputs the Nth character of the infinite code built from the initial string. The first character is N=1.

The last character of the string is moved to the first position and then the resulting string is appended to the previous string in each iteration.

SAMPLE INPUT:

COW 8

SAMPLE OUTPUT:

C

In this example, the initial string COW expands as follows:

COW -> COWWCO -> COWWCOOCOWWC

                 12345678

The problem is that for large values of N eg.10⁷, the code crashes. Can anyone please suggest methods to remove the error.

#include<bits/stdc++.h>
using namespace std;

char sCowCode(string str, long long int N){

    long long int a = str.length();

    if(N < a){
        return str[N-1];
    }

    while(2*a < N + 1)
        a *= 2;

    if(a == N)
        return sCowCode(str, a-1);

    return sCowCode(str, N - a -1);
}



int main(){

    long long int N;
    string str;

    cin >> str;
    cin >> N;


    char a = sCowCode(str, N);

    cout<<a<<endl;
    cin.get();
    cin.get();

    return 0;
}