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