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=str(input(''))
b= ''
c = ''
d= ''
e=0
while a[e]!=" " :
    b=b+a[e]
    e=e+1
e=e+1
while a[e]!=' ' :
    c=c+a[e]
    e=e+1
e=e+1
while e<len(a) :
    d=d+a[e]
    e=e+1
f=int(b)
g=int(c)
h=int(d)
j=0
for i in range(1,h+1):
    j=j+i*f
print(j-g)