Number of arrangements for given n numbers - java

find the number of arrangements for given n numbers in which no number exists in exact position .formula for this?

class Program { public static int output1; public static void derangement(int input1) { int n = input1; int op = fact(n); double sum = 0; for (int k = 0; k <= n; k++) { sum += Math.Pow(-1, k) / fact(k); } output1 = Convert.ToInt32(op * sum); } public static int fact(int n) { int f = 1; for (int i = 2; i <= n; i++) { f *= i; } return f; } static void Main(string[] args) { derangement(3); Console.WriteLine(output1); Console.ReadLine(); } } }