题解 | #牛牛的排列数#

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        long n = scan.nextLong();
        long m = scan.nextLong();
        if(n<m) { 
            System.out.println(0);
            return;
        }
        System.out.println(jc(n) / jc(n-m));  // 排列数的公式
    }
    public static long jc(long m) {
        long tmp = 1;
        for (int i=2;i<=m;i++) {
            tmp *= i;
        }
        return tmp;
    }
}