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();
        int t = scan.nextInt();
        scan.close();
        System.out.println(Math.round(((0.5*a*t)+(0.5*b*t))%(Math.pow(10,9)+7)));
    }
}