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();
        int result = (a*t)/2 + (b*t)/2;
        BigInteger big = new BigInteger("1000000007");
        BigInteger res = new BigInteger(String.valueOf(result));
        res = res.mod(big);
        System.out.println(res);
    }
}