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.
def calculate(k,n,w):
    money = n
    first_cost = k
    want_banana = w
    total_cost = 0
    for i in range(1,want_banana+1):
        total_cost += i * first_cost
    money_needed_to_ask_from_friend = total_cost - money
    if money_needed_to_ask_from_friend == 0:
        return 0
    else:
        return money_needed_to_ask_from_friend
    

input_string = input()
input_list = input_string.split()
k, n, w = map(int, input_list)

money_needed = calculate(k,n,w)
print(money_needed)
    




        
        