import java.io.*;
import java.util.*;
public class ExperimentOne {
    public static void main(String args[]) {
        System.out.println("输入n");
        Scanner A = new Scanner(System.in);
        int n = A.nextInt();
        System.out.println("输入m");
        Scanner B = new Scanner(System.in);
        int m = B.nextInt();
        chengji count=new chengji();
        try {
            count.sum(n,m);
            int sum = count.Sum();
            System.out.println("总数P:"+sum);
        }
        catch(Wrong f )
        {
            System.out.println("-1");
            System.out.println(f.Message());
        }
    }
}
class Wrong extends Exception {
    String FALSE;
    public Wrong(String m) {
        FALSE = m;
    }
    public String Message() {
        return FALSE;
    }
}
class chengji {
    int sum, x, y;
    public void sum(int a, int b) throws Wrong {
        if (a <= 0 || b <0 || a < b) {
            throw new Wrong("");
        }
        y = leicheng(a);
        x = leicheng(a - b);
    }
    public int leicheng(int e){
        sum=1;
        for(int i=1;i<=e;i++){
            sum=sum*i;
        }
        return sum;
    }
    public int Sum() {
        return (y/x);
    }
}