求Cmn如此

package com.company;
import java.util.Random;
import java.util.Scanner;
public class Main {
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        System.out.println("输入 m");
        int m = in.nextInt();
        System.out.println("输入 n");
        int n = in.nextInt();
        double num = 0;
        num = 1.0*function(n)/ function(m);
        System.out.println(num);
    }
            public static int function(int a){
               int sum = 1;
                for(int i = 1;i<=a;i++){
                    sum*=i;}
                return sum;}
    }