C++ :

#include <iostream>
using namespace std;
int a,b;
int main()
{
    cin>>a>>b;
    int c;
    c=b-2;
    b=0;
    for (int i=1;i<=c;i++) b+=i;
    c=a;a=1;
    for (int i=1;i<=c;i++) a+=i;
    cout<<(a-b);
    return 0;
}

Pascal :

var  n,p,t,i:longint;
begin
  readln(n,p);
  t:=2*p;
  for i:=p+1 to n do
    t:=t+i;
  writeln(t);
end.