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) {
        Scanner in = new Scanner(System.in);
        int a = in.nextInt();
        int b = in.nextInt();
        int t = in.nextInt();
        double pop = ((double)a + (double)b) / 2d;
        pop = (pop * (double)t) % (1000000007);
        System.out.println((int)pop);
    }
}