Combination

#include<bits/stdc++.h>
#define ll unsigned long long int
using namespace std;

ll combination(ll n, ll k)
{
    if(k == 0)
        return 1;
    if(k>n)
        return 0;
    if(k*2 > n)
        k = n - k;

    ll r = n, i;

    for(i=2;i<=k;i++)
    {
        r *= (n-i+1);
        r /= i;
    }

    return r;
}

int main()
{
    ll n, m;
    //where n is n and m is r = nCr;
    while(cin >> n >> m)
    {
        ll r = combination(n, m);
        cout << r << endl;
    }
    return 0;
}