package No400番台;

import java.util.Scanner;

public class Q411 {
	public static void main(String[] args) {
		new Q411().solver();
	}

	void solver() {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int K = sc.nextInt();
		if (K == 1&&N==2) {
			System.out.println(0);
			return;
		}
		long ans = 0;
		for (int i = 0; i <= N - K; i++) {
			ans += nCk(N - K, i);
		}
		System.out.println(ans);
	}

	static long nCk(int n, int k) {
		if (n < k)
			return 0;
		else {
			return (fact(n) / fact(n - k) / fact(k));
		}
	}

	static long fact(int n) {
		long ans = 1;
		for (int i = 1; i <= n; i++)
			ans *= i;
		return ans;
	}
}