import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {

    public static void main(String[] args) {
        /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
        Scanner sc=new Scanner(System.in);
        double a=sc.nextInt()*0.5;
        double b=sc.nextInt()*0.5;
        int t=sc.nextInt();
        System.out.println((int)(((a*t)+(b*t))%Math.pow(10,9)));
    }
}