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 in=new Scanner(System.in);
        int a,b,t;
        a=in.nextInt();
        b=in.nextInt();
        t=in.nextInt();
        in.close();
        int sum=a+b;
        int prod=(t*sum)/2;
        int exp=1000000007;
          int result=prod%(exp);
         //uble i = Math.round(result);
        System.out.println(result);
    }
}