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

public class Solution {

    public static void main(String[] args) {
        Scanner in=new Scanner(System.in);
        int a,b,t;
        a=in.nextInt();
        b=in.nextInt();
        t=in.nextInt();
        double k=Math.pow(10.0, 9)+7.0;
        double res=((0.5*((double)a)*((double)t))+(0.5*((double)b)*((double)t)))%k;
        System.out.println((int)res);    
    }
}