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.
# A. Soldier and Bananas

inputUser = input().split()
k, n, w = int(inputUser[0]), int(inputUser[1]), int(inputUser[-1]) 
result = 0
# the cost of the first banana,
# initial number of dollars the soldier has and number of bananas he wants.

for i in range(1, w+1):
  if ((i * k) <= n):
    n -= (i * k)
  elif ((i * k) > n):
    result += (i * k) - n
    n = 0

print(result)