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. */
        int a,b,c;
        double t,sum=0.0,ans=0.0;
        Scanner scan =new Scanner(System.in);
        a=scan.nextInt();
        b=scan.nextInt();
        t=scan.nextLong();
        sum=0.5*t*(a+b);
        ans=sum%(Math.pow(10.0,9.0)+7);
        c=(int) ans;
        System.out.println(c);
        scan.close();
    }
}