题目：

Description

A soldier wants to buy w bananas in the shop. He has to pay k dollars for the first banana, 2k 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?

Input

The first line contains three positive integers k, n, w (1  ≤  k, w  ≤  1000, 0 ≤ n ≤ 10⁹), the cost of the first banana, initial number of dollars the soldier has and number of bananas he wants.

Output

Output one integer — the amount of dollars that the soldier must borrow from his friend. If he doesn't have to borrow money, output 0.

题目大意：第i个香蕉的价格是ik元，求总共的价钱

算法用等差数列的求和公式：（1+i）*i/2；

代码：

1 #include
       
         2 using namespace std; 3 int main() 4 { 5     int k,w,n,sum,b,price; 6     while(cin>>k>>n>>w) 7     { 8         if(k>=1&&w<=1000&&n>=0&&n<=1000000000) 9     {10         sum=(1+w)*w/2;11         price=sum*k;12         if(price>n)13         {14             b=price-n;15             printf("%d\n",b);    16         }17         else printf("0\n");18     }19     }20     return 0;21 }