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
input = sys.stdin.readline
import math as m
import heapq as hq
import time

def inp():
    return list(map(int,input().split()))

mod=1000000007

def binpow(a,b,m):
    res=1
    while b>0:
        if b&1:
            res=(res*a)%m
        a=(a*a)%m
        b>>=1
    return res%m

def phi(n):
    res=n
    for i in range(2,int(m.sqrt(n))+1):
        if n%i==0:
            while (n % i) == 0:
                n//=i
            res-=(res//i)
    if n>1:
        res-=(res//n)
    return res

def mod_inv(q):
    return binpow(q,mod-2,mod)

def lcm(p,q):
    return (abs((p*q)//m.gcd(p,q)))

def sievespf(n):
    spf = [i for i in range(n + 1)]
    p = 2
    while p * p <= n:
        if spf[p] == p:
            for i in range(p * p, n + 1, p):
                if spf[i] == i:
                    spf[i] = p
        p += 1 
    return spf

def prime_factors(n):
    fac=[]
    while n%2==0:
        n//=2
        if 2 not in fac:
            fac.append(2)
    for i in range(3,int(m.sqrt(n))+1,2):
        while n%i==0:
            n//=i
            if i not in fac:
                fac.append(i)
    if n>2:
        if n not in fac:
            fac.append(n)
    return fac

def st(n):
    return (n*(n+1))//2

def solve():
    k,w,n=inp()
    total_cost=st(n)*k
    print(total_cost-w)
    return 0

#for _ in range(int(input())):
#    start_time = time.time()
solve()
#    end_time = time.time()
#    execution_time = end_time - start_time
    #print(f"Test case executed in: {execution_time:.6f} seconds")