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. */
        int a,b,t;
        long nov;
        Scanner sc = new Scanner(System.in);
        a = sc.nextInt();
        b = sc.nextInt();
        t = sc.nextInt();
        
        nov =(long) ((0.5 * a * t) +( 0.5 * b * t))%(1000000000+7);
            
        System.out.println(nov);
        
        
        
    }
}