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

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 s=new Scanner(System.in);
        int a=s.nextInt();
        int b=s.nextInt();
        int t=s.nextInt();
        double temp=a*0.5*t+ b*0.5*t;
        temp=temp%1000000007;

        System.out.println(Math.round(temp)); 
     
       
    }
}