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

public class Solution {

    public static void main(String[] args) {
        int t,a,b,i,op;
        int ip[]=new int[3];
        double m=Math.pow(10,9)+7;
        Scanner sc=new Scanner(System.in);
        for(i=0;i<3;i++)
            {
            ip[i]=sc.nextInt();
        }
        a=ip[0];
        b=ip[1];
        t=ip[2];
        op=(int)(0.5*t*(a+b));
        op%=m;
        System.out.println(op);
    }
}