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.
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Wed Sep 13 17:22:41 2023

@author: huangxiaoyuan
"""

k,n,w=input().split()
k=int(k)
n=int(n)
w=int(w)
i=1
sum=0
if k>=1 and w<=1000 and 0<=n<=10**9:
    k0=k
    while i<=w:
        sum+=k
        k+=k0
        i+=1
    sum1=sum-n
print(sum1)
    
        
