Thornhill Computer Club 2024 - April Contest - Problem 1

An odd country taxes customers in stores for buying certain amounts of items, in order to encourage bulk buying:

Given the number of items purchased by a single customer and each of their prices, determine the final cost to buy the items.

Input Specification

The first line will contain an integer ~N~ ~(1 \le N \le 1000)~ representing the number of different items that the customer will buy.

The next ~N~ lines will consist of 2 space-separated integers, ~K~ ~(1 \le K \le 1000)~, the cost of the item, and ~L~ ~(1 \le L \le 1000)~, the amount of the item that the customer will buy.

Output Specification

On a single line, output the cost of the entire purchase.

Sample Input 1

2
5 8
8 1

Sample Output 1

49.0

Explanation for Sample Output 1

The customer wants to buy 8 $5 items, and they also want to buy 1 $8 item. This adds up to $40 + $8 = $48. Additionally, the customer wants to buy a total 2 items, so the store will charge $1 on top, so the final cost is $49.