every body handshakes all else his couple
so
n*(n-1) div 2
-
k (couples doesnt handshake eachother)!
-->

Var
  n,k:integer;
begin
  readln(n,k);
  writeln(((n*(n-1)) div 2) -k);
end.

Aidin_n7

n(n-1)/2-k。。。

What is The Autor Thinking?
What's the meaning of the spliting group numbers?
It just piss me off.

Yep, this is the one way for solution.

I've met problem like that earlier. Statements consist kinda "you need to get very-very much operation with binary code of number", but after some tests we understood, that solution was kinda "ans = n*(-1)".

And it's good to have a short cut in problems for clever men;)