Problem:
Heat Stroke

　
We have had record hot temperatures this summer. To avoid heat stroke, you decided to buy a quantity of drinking water at the nearby supermarket. Two types of bottled water, 1 and 0.5 liter, are on sale at respective prices there. You have a definite quantity in your mind, but are willing to buy a quantity larger than that if: no combination of these bottles meets the quantity, or, the total price becomes lower.

Given the prices for each bottle of water and the total quantity needed, make a program to seek the lowest price to buy greater than or equal to the quantity required.

Input

The input is given in the following format.

$A$ $B$ $X$

The first line provides the prices for a 1-liter bottle $A$ ($1\leq A \leq 1000$), 500-milliliter bottle $B$ ($1 \leq B \leq 1000$), and the total water quantity needed $X$ ($1 \leq X \leq 20000$). All these values are given as integers, and the quantity of water in milliliters.

Output

Output the total price.

Sample Input 1

180 100 2400

Sample Output 1

460

Sample Input 2

200 90 2018

Sample Output 2

450

Solution in JavaScript (judge verdict: Accepted)

var input = require('fs').readFileSync('/dev/stdin', 'utf8');
var [a,b,x]=input.trim().split(" ").map(Number);
var min=Infinity;
for(var i=0;i<=20;i++){
   for(var j=0;j<=40;j++){
      if(i*1000+j*500>=x)min=Math.min(min,i*a+j*b);
   }
}
console.log(min);