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);
        double growthRate1 = in.nextInt();
        double growthRate2 = in.nextInt();
        double milliSeconds = in.nextInt();

        final double probability = 0.5;

        double returnValue = ((growthRate1+growthRate2)*probability*milliSeconds)%(Math.pow(10, 9) + 7);
        int castValue = (int) Math.round(returnValue);
        System.out.print(castValue);
    }
}