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 sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        long t = sc.nextLong();

        long modulo = (long) (Math.pow(10, 9) + 7);
        long result = (long) ((0.5 * t * (a + b)) % modulo);

        System.out.println(result);
        sc.close();
    }
}