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


def iinp():
    return int(sys.stdin.readline().strip())


def linp():
    return list(map(int, sys.stdin.readline().strip().split()))


def lsinp():
    return sys.stdin.readline().strip().split()


def digit():
    return [int(i) for i in (list(sys.stdin.readline().strip()))]


def char():
    return list(sys.stdin.readline().strip())


from bisect import bisect_left, bisect_right
from collections import defaultdict


def summ(n):
    return sum(int(i) for i in list(str(n)))


from math import ceil


def solve():
    k, n, w = linp()
    ans = k * (w * (w + 1) // 2) - n
    if ans > 0:
        print(ans)
    else:
        print(0)


q = 1
for _ in range(q):
    solve()
