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.
banana_price , dollars_he_has , number_of_bananas = map(int , input().split())
instead_0f_num_of_bananas = number_of_bananas
for x in range(1 , number_of_bananas+1):
    if instead_0f_num_of_bananas > 0 :
        price = x*banana_price
        dollars_he_has = dollars_he_has - price
        instead_0f_num_of_bananas -= 1
    else :
        break
if dollars_he_has >= 0 :
    output = 0 
else :
    output = abs(dollars_he_has)
print(output)
