Codeforces Round #539 (Div. 2) A. Sasha and His Trip(思维)

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

int main()
{
  int n,m;
  cin>>n>>m;
  n --;
  if(m >= n){
    cout<<n<<endl;
    return 0;
  }
  int ans = m;
  int pos = n - m;
  for(int i=1;i<=pos;i++){
    ans += i + 1;
  }
  cout<<ans<<endl;
  return 0;
}