Multiply big integers under big modulo

#include<bits/stdc++.h>

#define ll unsigned long long int

using namespace std;

ll mod(ll a, ll b, ll m)

{

    ll r = 0;

    a = a%m;

    while(b > 0)

    {

        if(b&1)         /*** b%2 == 1 ***/

            r = (r + a)%m;

        b = (b >> 1);      /*** b = b/2 ***/

        a = (a+a)%m;

    }

    return r;

}

int main()

{

    ll a, b, m;

    cout << "Enter First Number :: ";

    cin >> a;

    cout << "Enter Second Number :: ";

    cin >> b;

    cout << "Enter Mod Number :: ";

    cin >> m;

    cout << "After Modulo Operation :: ";

    cout << mod(a, b, m) << "\n";

    return 0;

}