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. Soldier and Bananas"""
k,n,w=map(int, input().split())
#w num of bannana 
#k the cost for first bannana
#n money he has
#how much need to borrow money from his friend
i=1
c=0
v=0
while i <=w:
    s=i*k
    v+=s
    i+=1
    
c+=v

if n>=c:
    print(0)
else:
    l=n-c
    print(-l)
        