import java.io.*;
import java.util.*;

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 scan = new Scanner (System.in);
        int a = scan.nextInt();
        int b = scan.nextInt();
        long millisecond = scan.nextLong();
        long c = Math.round(0.5*10);
        long fin = ((long)a * millisecond) + ( (long)b * millisecond) ;
        fin = (fin/2);
            fin = fin %(long) ( 100000007);
        System.out.println(fin);
        
    }
}