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

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);
        int a = in.nextInt();
        int b = in.nextInt();
        long t = in.nextInt();
        int ans = (int)((0.5 * a * t + 0.5 * b * t) % 1000000007);
        System.out.println(ans);
    }
}