using System;
using System.Collections.Generic;
using System.IO;
class Solution {
    static void Main(String[] args) {
        /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution */
        var s = Console.ReadLine().Split(' ');
        int a = Convert.ToInt32(s[0]);
        int b = Convert.ToInt32(s[1]);
        int t = Convert.ToInt32(s[2]);
        double prob = 0.5*t*(a+b) % 1000000007;
        Console.WriteLine(prob);
    }
}