#include "bits/stdc++.h"
#define _CRT_SECURE_NO_WARNINGS
#define rep(i,n) for(int i = 0;i < n;i++)
#define REP(i,n,k) for(int i = n;i < k;i++)
#define P(p) cout << (p) << endl;
#define sP(p) cout << setprecision(15) << fixed << p << endl;
#define Pi pair<int,int>
#define IINF 1e9
#define LINF 1e18
#define vi vector<int>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
int dx[] = { 0, 1,0,-1 };
int dy[] = { 1, 0,-1,0 };

ull gcd(ull a, ull b) {
	if (b > a)swap(a, b);
	if (b == 0) return a;
	return gcd(b, a%b);
}

int comb(int n, int k) {
	int ans = 1;
	for (int i = 0; i < min(k, n - k); i++) {
		ans *= n - i;
		ans /= i + 1;
	}
	return ans;
}

void solve() {
	int n, k;
	cin >> n >> k;
	int sum = 0;
	if (k != 1) {
		REP(i, 1, n - k + 1) {
			sum += comb(n - k, i);
		}
		sum++;
	}
	else {
		sum = n - 2;
	}
	P(sum);
}

int main() {
	solve();
	return 0;
}