import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {
    int sum=0;
    int rep(int i,int j,int k)
        {
        
        sum=(int)(0.5*i*k+0.5*j*k);
        return sum;
    }

    public static void main(String[] args) {
  Scanner sc=new Scanner(System.in);
     int time,a,b;
    a=sc.nextInt();
    b=sc.nextInt();
    time=sc.nextInt();
    
        
        
        /* Enter your code h
        ere. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
    Solution s=new Solution();
        int l=s.rep(a,b,time);
        int  k=(int)(l%(Math.pow(10,9)+7));
    System.out.print(k);
    }
}