import java.io.*;
import java.util.*;
import java.lang.*;

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. */
        long m=(long)Math.pow(10,9)+7;
        Scanner s = new Scanner(System.in);
        long a,b,t;
        a=s.nextLong();
        b=s.nextLong();
        t=s.nextLong();
        long expect = (long)(((0.5*t)*(a+b))%m);
        System.out.println(expect);
    }
}