codeforce - Sasha and His Trip

#include<bits/stdc++.h>
using namespace std;
#define fastread()(ios_base::sync_with_stdio(false),cin.tie(NULL));
int main()
{
    int n,v;
    scanf("%d %d",&n,&v);
    if(v>=n)printf("%d\n",n-1);
    else if(v-1==n)
    {
        printf("%d\n",v);
    }
    else
    {
        int i,res=0, pos;
        for(i=1;i<=n-v;i++)
        {
           if(i==1)res=i*v;
           else
           {
               res+=i;
           }
        }
        printf("%d\n",res);
    }

}