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 in = new Scanner(System.in);

        long p1 = in.nextInt();
        long p2 = in.nextInt();
        long time = in.nextInt();
        long mod = (long) (Math.pow(10, 9) + 7);
        long answer = ((p1*time/2)+(p2*time/2))%mod;

        System.out.println(answer);

    }
}