using System;
using System.Collections.Generic;
using System.Linq;

class Program {
    static string ReadLine() { return Console.ReadLine(); }
    static int ReadInt() { return int.Parse(ReadLine()); }
    static int[] ReadInts() { return ReadLine().Split().Select(int.Parse).ToArray(); }
    static string[] ReadStrings() { return ReadLine().Split(); }

    static long Combination(int n, int r) {
        long num = 1;
        long den = 1;
        for (int i = 0; i < r; i++) {
            num *= n - i;
            den *= r - i;
        }
        return num / den;
    }

    static long Calc(int n, int k) {
        if (k == 1) return 0;

        long sum = 0;
        int p = n - k;
        for (int i = 0; i <= p; i++) {
            sum += Combination(p, i);
        }
        return sum;
    }

    static void Main() {
        var xs = ReadInts();
        int n = xs[0], k = xs[1];
        Console.WriteLine(Calc(n, k));
    }
}