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.
from math import ceil


def i():
    return(int(input()))
def l():
    return(list(map(int,input().split())))
def s():
    s = input()
    return(s)
def sl():
    s = input()
    return(list(s[:len(s) - 1]))
def m():
    return(map(int,input().split()))

k, n, w = m()
count = 0

for i in range(1, w + 1):
    count += k * i
print(abs(n - count))