#define _USE_MATH_DEFINES #include <cstdio> #include <iostream> #include <sstream> #include <fstream> #include <iomanip> #include <algorithm> #include <cmath> #include <complex> #include <string> #include <vector> #include <array> #include <list> #include <queue> #include <stack> #include <set> #include <map> #include <bitset> #include <numeric> #include <limits> #include <climits> #include <cfloat> #include <functional> #include <iterator> #include <memory> #include <regex> using namespace std; long long sum(int a) { return a * (a + 1LL) / 2; } int main() { int n, k; cin >> n >> k; long long ans = 1 + sum(n) - sum(n-k); cout << ans << endl; return 0; }