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

input = [int(x) for x in sys.stdin.readline().split(" ")]
k = int(input[0])
n = int(input[1])
w = int(input[2])

if w % 2 == 0:
    total = (1 + w)*(w/2)*k
else:
    total = ((1 + w)*((w - 1)/2) + (w + 1)/2)*k

borrow = n - total
if borrow > 0:
    borrow = 0

sys.stdout.write(str(int(abs(borrow))))