nCk_expansion.cpp

#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define Min(a,b) (a<=b)? a : b

ll nCr(ll n, ll r)
{
    if (r==0) return 1;
    else return nCr(n-1,r-1) * n / r;
}

int main()
{
    ll n,r,m;
    while (cin >> n >> r)
    {
        cout << nCr(n, Min(r, n-r)) << endl;
    }
    return 0;
}