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.
number = input()
scores = []
scores.append(int(number[0]))
for x in range(len(number)-1):
    if number[x] != " " and number[x+1] != " ":
        scores.append(int(number[x]+number[x+1]))
    elif number[x] != " " and number[x+1] == " " and number[x-1] == " ": 
        scores.append(int(number[x]))                  
    else:
        continue
scores.append(int(number[len(number)-1]))
k = int(scores[0]) #cost of first banana
n = int(scores[1]) #number of dollares
w = int(scores[2]) #number of bananas he want
cost = 0
for x in range(w+1):
    cost += x*k   
print(cost-n)


