A soldier wants to buy *w* bananas in the shop. He has to pay *k* dollars for the first banana, 2*k* 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  ≤  *k*, *w*  ≤  1000, 0 ≤ *n* ≤ 10<sup class="upper-index">9</sup>), the cost of the first banana, initial number of dollars the soldier has and number of bananas he wants.
# -*- coding: utf-8 -*-
"""
Created on Mon Sep 18 17:09:47 2023

@author: josea
"""
bananInfo = input()
 
k = int(bananInfo.split()[0])
n = int(bananInfo.split()[1])
w = int(bananInfo.split()[2])
totP = 0
baseP = 0
for i in range(w):
    totP = baseP + (i+1)*k
    baseP = totP
borrow =    totP - n
print(borrow)