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 = sys.stdin.readline

def in2i():
    '''
    input to int
    '''
    return int(input())

def in2il():
    '''
    intput to int list
    '''
    return (list(map(int, input().split())))

def s2cs():
    '''
    string to chars
    '''
    s = input()
    return (list(s[:len(s)]))

k, n, w = in2il()
s = 0
for i in range(1, w + 1):
    s += i * k
if n > s:
    print(0)
else:
    print(s - n)