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) {
       int a,b,t,n,n1;
        Scanner scan=new Scanner(System.in);
        a=scan.nextInt();
        b=scan.nextInt();
        t=scan.nextInt();
    
    n=((a*t)+(b*t))/2;
      n1=n%(1000000007);
        
        System.out.print(n1);
    }
}