#include<bits/stdc++.h>
using namespace std;
using ll = long long;

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<endl
#define p2(s, t) cout << (s) << " " << (t) << endl
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << endl
#define p_yes() p("Yes")
#define p_no() p("No")

const int N_MAX = 21;
ll pascal[N_MAX][N_MAX] = {};

void calc_pascal(){
    FOR(i, 0, N_MAX){
        pascal[i][0] = 1;
    }
    pascal[1][1] = 1;

    FOR(i, 2, N_MAX){
        for(int j=1; j<=i; j++){
            pascal[i][j] = pascal[i-1][j] + pascal[i-1][j-1];
        }
    }
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    // input
    ll N, K;
    cin >> N >> K;

    calc_pascal();

    if(K==1){
        p(N-2);
    }
    else{
        ll sum = 0;
        FOR(i, 0, N_MAX){
            sum += pascal[N-K][i];
        }
        p(sum);
    }
    
    return 0;
}