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.
import sys
def I(): return int(sys.stdin.readline().rstrip())
def LI(): return list(map(int, sys.stdin.readline().rstrip().split()))
def MI(): return map(int, sys.stdin.readline().rstrip().split())
def SI(): return sys.stdin.readline().rstrip()


def main():
    k,n,w = MI()
    cost = 0
    for i in range(1,w+1):
        cost += i*k
    if cost > n:    
        print(cost - n) 
    else:    
        print(0)
        

if __name__ == "__main__":
    main()