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.
class solution:
    def __init__(self):
        self.list1 = []
        self.r = 0
        self.w = 0
        self.k = 0
        self.n = 0
        
    def compute(self):
        sum1 = (self.w*(self.w+1))/2
        amount = self.k*sum1
        self.r = amount - self.n
    
    def change(self):
        self.k = int(self.list1[0])
        self.n = int(self.list1[1])
        self.w = int(self.list1[2])
 
    def get_input(self):
        str1 = input()
        temp = str1.split()
        self.list1 = temp
        del temp
    
    def check(self):
        if self.r <= 0:
            self.r = 0
 
    def show(self):
        print(int(self.r))
        
    def execute(self):
        self.get_input()
        self.change()
        self.compute()
        self.check()
        self.show()
    
def main():
    s1 = solution()
    s1.execute()
    
if __name__ == '__main__':
    main()  