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

public class Solution {
	
	public static void main(String s[]){
		
		Scanner sc = new Scanner(System.in);
		
		int a = sc.nextInt();
		
		int b = sc.nextInt();
		
		int t  = sc.nextInt();
		
		int num = (int)((0.5 * a * t) + (0.5 * b *t));
		
		System.out.println((int)(num % (Math.pow(10, 9) + 7)));
		
	}

}