content: /*
  Write a program that asks the user for a number N and a choice C. And then give them the possibility to choose between computing the sum and computing the product of all integers in the range 1 to N (both inclusive).
  If C is equal to -
  1, then print the sum
  2, then print the product
  Any other number, then print '-1' (without the quotes)
  
  Input format :
  Line 1 : Integer N
  Line 2 : Choice C
  
  Output Format :
  Sum or product according to user's choice
  
  Constraints :
  1 <= N <= 12
  
  Sample Input 1 :
  10
  1
  Sample Output 1 :
  55
  
  Sample Input 2 :
  10
  2
  Sample Output 2 :
  3628800
  
  Sample Input 3 :
  10
  4
  Sample Output 3 :
  -1
  */
  
  import java.util.*;
  public class SumOrProduct {
  	
  	public static void main(String[] args) {
  		// Write your code here
  		Scanner scan = new Scanner(System.in);
          int N=scan.nextInt();
          int C=scan.nextInt();
          int ans=1;
          switch(C)
          {
              case 2:
              for (int i=2;i<=N;i++)
              {
                  ans=ans*i;
              }
              break;
              
              case 1:
              for (int i=2;i<=N;i++)
              {
                  ans=ans+i;
              }
              break;
              
              default:
              ans=-1;
              break;
          }
          
          System.out.println(ans);
  	}
  }
  