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 = 1
info = input(). split()
cost = int(info[0])
cash = int(info[1])
need_banans = int(info[2])
have_bananas = 0

while have_bananas != need_banans:
    cash -= cost*a
    a += 1
    have_bananas +=1
if cash < 0:
    print(abs(cash))
else:
    print(0)