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.
import sys

data = sys.stdin.readline().strip().split()
base = [int(i) for i in data]
price_first_banan = base[0]
cash = base[1]
sum_buy_bananas = base[2]

step = 1

must_pay = 0

while sum_buy_bananas != 0:
    i = price_first_banan * step
    must_pay += i
    step += 1
    sum_buy_bananas -= 1

he_lend = must_pay - cash

sys.stdout.write(str(he_lend))