namespace etc { static class Etc { static BigInteger combination(int n, int r) { return (factorial(n) / factorial(r) / factorial(n - r)); } static Dictionary f5Cache = new Dictionary(); static int f5(BigInteger n) { if (f5Cache.ContainsKey(n)) return f5Cache[n]; int r = 0; for (int x = (int)Double.Parse(BigInteger.Log(n, 5).ToString()); x > 0; x--) { if (n % BigInteger.Pow(5, x) == 0) { r = x; b..