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. */
         long a,b;
        double n,n1;
        long t;
        Scanner scan=new Scanner(System.in);
        a=scan.nextLong();
        b=scan.nextLong();
        t=scan.nextLong();
    
    n=((0.5*a*t)+(0.5*b*t));
      n1=n%(Math.pow(10,9)+7);
       /* long t1=Math.pow(10,9)+7;
         BigInteger bi1, bi2, bi3;

        bi1 = new BigInteger(n);
        bi2 = new BigInteger(t1);

        // perform mod operation on bi1 using bi2
	bi3 = bi1.mod(bi2);
        //Double obj = new Double(n1);
     //int i = obj.intValue();*/
        
        System.out.print(Math.round(n1));
        
    //System.out.print(bi3);
    }
}