using System;
using System.Linq;

class Program
{
    static void Main()
    {
        var a = Console.ReadLine().Split().Select(i => int.Parse(i)).ToArray();
        if(a[1]==1) 
        {
            var k = a[0] - 2;
            if(k==0) Console.WriteLine(0);
            else
            {
            	int p = (int)Math.Pow(2,k);
                int t = a[0]-3;
                int count =0;
                for(;t>0;t--)count+=t;
                Console.WriteLine(count+p);
            }
        }
        else Console.WriteLine(Math.Pow(2,a[0]-a[1]));
    }
}