import java.io.*;
import java.util.*;

public class Solution {

    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        
        long arate=sc.nextLong();
        long brate=sc.nextLong();
        long time=sc.nextLong();
        
        long finalvalue=(arate*time/2 + brate*time/2)%(10^9+7);
        System.out.println(finalvalue);
    }
}