import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {

    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        int a = s.nextInt();
        int b = s.nextInt();
        long t = s.nextLong();
        long part1 = (a * t);
        part1 /= 2;
        long part2 = (b * t);
        part2 /= 2;
        long result = part1 + part2;
        result = result % 1000000007;
        System.out.println(result);
    }
}