CF546A Soldier and Bananas

A soldier wants to buy $ w $ bananas in the shop. He has to pay $ k $ dollars for the first banana, $ 2k $ dollars for the second one and so on (in other words, he has to pay $ i·k $ dollars for the $ i $ -th banana). He has $ n $ dollars. How many dollars does he have to borrow from his friend soldier to buy $ w $ bananas?

The first line contains three positive integers $ k,n,w $ ( $ 1

Output one integer — the amount of dollars that the soldier must borrow from his friend. If he doesn't have to borrow money, output $ 0 $ .