using System.Numerics;
BigInteger Binomial(int n, int k) => k < 0 || k > n ? 0 : k == 0 ? 1 : k * 2 > n ? Binomial(n, n-k) : (Binomial(n, k-1) * n)/k;
BigInteger f(int r)
{
    BigInteger result = 0;
    for(int i = 0; i <= r; ++i) result += Binomial(30, i) * Binomial(20, r-i);
    return result;
}
Console.WriteLine(f(20));// 3,347,717,751,371,268
Console.WriteLine(f(25));// 50,335,833,680,826,500