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

public class Solution {
    static Scanner in;
    public static void main(String[] args) {
      in=new Scanner(System.in);
     int a= in.nextInt();
      int b= in.nextInt();
      int t = in.nextInt();//time in millisecs
      int ans= 0;
      for(int m=0; m< t; m++)
      {
        ans+= .5* (a+b);
      }
      System.out.println (ans%1000000007);
    }
}