How to Calculate 2^x mod n = 1 in less than 1 second

I want to write the program that Calculate 2^x mod n = 1 we have n is an integer but, we should calculate x.I wrote the code but my code work too slow in big n.Can you suggest me a good way that work less than 1 second to solve this problem.
here is my code:

#include <iostream>
#include <cmath>
using namespace std;
int main()
{
    long long int n,cntr=1,cheak;
    cin >> n;
    while (1)
    {
        if (n % 2 == 0)
        {
            break;
        }
        cheak=pow(2, cntr);
        if (cheak % n == 1)
            break;
        cntr++;
    }
    cout << cntr << endl;
}