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

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[]=new int[3];
        double sum=0;
        for(int i=0;i<3;i++)
            a[i]=s.nextInt();
        for(int i=1;i<=a[2];i++)
            sum=sum+(0.5*a[0]*i)+(0.5*a[1]*i);
       //  System.out.println(sum);
        sum=sum% (java.lang.Math.pow(10,9)+7);
        System.out.println((int)sum);
        
            
    }
}