import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int k = sc.nextInt();
        if (n == 2 && k == 1) {
            System.out.println(0);
            return;
        }
        long[][] comb = new long[n + 1][n + 1];
        for (int i = 0; i <= n; i++) {
            for (int j = 0; j <= i; j++) {
                if (j == 0 || j == i) {
                    comb[i][j] = 1;
                } else {
                    comb[i][j] = comb[i - 1][j - 1] + comb[i - 1][j];
                }
            }
        }
        long total = 1;
        if (k == 1) {
            for (int i = 1; i < n - 2 && i <= n - k; i++) {
                total += comb[n - k][i];
            }
        } else {
            for (int i = 1; i < n - 1 && i <= n - k; i++) {
                total += comb[n - k][i];
            }
        }
        System.out.println(total);
    }
}