nCr

#include<bits/stdc++.h>
using namespace std;
int nCrModp(int n, int r, int p)
{
    int C[r+1];
    memset(C, 0, sizeof(C));
    C[0] = 1;
    for (int i = 1; i <= n; i++)
    {
        for (int j = min(i, r); j > 0; j--)
            C[j] = (C[j] + C[j-1])%p;
    }
    return C[r];
}
int main()
{
    int n = 10, r = 2, p = 13;
    cout << nCrModp(n, r, p);
    return 0;
}