#include #include #include #include #include #include #include #include #include #include #include #include #include using namespace std; #define N (1000000000+7) //#define N 998244353 #define INF 1e16 typedef long long ll; typedef pair P; typedef pair Q; const int inf = (int)1e9; ll gcd(ll a, ll b) { if (b > a) { ll tmp = b; b = a; a = tmp; } if (a%b == 0)return b; else return gcd(b, a%b); } int main(void){ ll n,k; cin>>n>>k; ll ans = 0; for(ll i=0;i<=k;i++){ if(i==0)ans++; else{ ans+=n-i+1; } } cout<