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 in = new Scanner(System.in);
        int a = in.nextInt();
        int b = in.nextInt();
        long t = in.nextInt();
        
        long num = Math.round(0.5*a*t + 0.5*b*t);
        num = num%(1000000007);
        System.out.println(num);
        
    }
}