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) {
        /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
        Scanner input = new Scanner(System.in);
        double a = input.nextDouble();
        double b = input.nextDouble();
        int t = input.nextInt();
        int expected = 1;
        //for(int i = 0; i < t; i++){
            expected = ((int)(0.5*a*t) + (int)(0.5*b*t))%(int)((Math.pow(10,9))+7);
        //}
        System.out.print(expected);
    }
}