#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
//#include<cctype>
#include<climits>
#include<iostream>
#include<string>
#include<vector>
#include<map>
//#include<list>
#include<queue>
#include<deque>
#include<algorithm>
//#include<numeric>
#include<utility>
//#include<memory>
#include<functional>
#include<cassert>
#include<set>
#include<stack>
#include<random>

const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, -1, 0, 1};
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef pair<int, int> pii;

ll comb[22][22];

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    for (int i = 0; i < 22; i++) {
        comb[i][0] = 1;
        for (int j = 1; j <= i; j++) {
            comb[i][j] = comb[i-1][j] + comb[i-1][j-1];
        }
    }
    int N, K;
    cin >> N >> K;
    ll ans = 0;
    if (K == 1) {
        for (int i = 1; i <= N-1; i++) {
            ans += comb[N-1][i-1];
            ans--;
        }
    } else {
        for (int i = 1; i <= N-K+1; i++) {
            ans += comb[N-K][i-1];
        }
    }
    cout << ans << endl;
    return 0;
}