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.
from sys import stdin, stdout
import math

MOD = 1000000007
INF = float('inf')

def read_int():
    return int(stdin.readline().rstrip())

def read_map():
    return map(int, stdin.readline().rstrip().split())

def read_lst():
    return list(map(int, stdin.readline().rstrip().split()))

def read_str():
    return stdin.readline().rstrip()

def write(*args, **kwargs):
    sep = kwargs.get('sep', ' ')
    end = kwargs.get('end', '\n')
    stdout.write(sep.join(map(str, args)) + end)

def ok(val):
    write("YES" if val else "NO")

# >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

def solve():
    k, n, w = read_map()

    c = 0
    for i in range(w):
        c += (i + 1) * k

    print(c - n if c > n else 0)

# <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

def main():
    for _ in range(1):
        solve()

if __name__ == "__main__":
    main()
