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.
#546A
def p(x):
    print(x)
def cost(k , n , w):
    summ = 0
    j = 1
    banans = 0
    duty = 0
    for _ in range(w):
        banans += j
        j += 1
    summ += k * banans
    if summ <= n:
        p(n - summ)
    else:
        p(summ - n)

#основа
    
s = str(input())
s = s.split(' ')
ww = int(s[2])
nn = int(s[1])
kk = int(s[0])
cost(kk , nn, ww)
