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 inp=new Scanner(System.in);
        int a,b;
        long t,res;
        a=inp.nextInt();
        b=inp.nextInt();
        t=inp.nextLong();
        res=(long)((((a+b)*t)/2))%(1000000007);
        System.out.println(res);
        inp.close();
        
    }
}