#include <iostream>
using namespace std;

typedef long long ll;

ll nCk(int n, int k) {
    k = min(k, n - k);
    if (k == 0) return 1;
    if (k == 1) return n;

    return nCk(n - 1, k - 1) * n / k;
}

int main() {
    int n, k;
    cin >> n >> k;

    if (n == 2 && k == 1) {
        cout << 0 << endl;
        return 0;
    }

    ll ans = 0;
    for (int len = 1; len < n; len++) {
        if (n - k < len - 1) break;
        ll ways = nCk(n - k, len - 1);
        ans += ways;
    }
    cout << ans << endl;

    return 0;
}