#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define llong long long

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

    int result;
    if(k == 1) result = n - 2;
    else if(n==k) result = 1;
    else {
        result = 1;
        for(int i=k+1; i<=n; i++) {
            result += n - i + 1;
        }
    }

    cout << result << endl;
}