/*
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 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
import java.io.*;
import java.util.*;

/**
 *
 * @author dakata
 */
public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws FileNotFoundException {
        // TODO code application logic here
        /*
        Scanner scanner = new Scanner(new FileInputStream("adunare.in"));
        PrintStream writer = new PrintStream("adunare.out");
                
        int a = Integer.parseInt(scanner.nextLine());
        int b = Integer.parseInt(scanner.nextLine());
        writer.println(a+b);
         */
        Scanner scanner = new Scanner(new FileInputStream("aria.in"));
        PrintStream writer = new PrintStream("aria.out");
        String line_1 = "";
        String line_2 = "";
        String[] cuv;
        String[] cuv_2;
        int sum = 0;
        int N = Integer.parseInt(scanner.nextLine());
        for (int i = 0; i < N / 2; i++) {

            line_1 = scanner.nextLine();
            line_2 = scanner.nextLine();
            cuv = line_1.split(" ");
            cuv_2 = line_2.split(" ");
            sum += Integer.parseInt(cuv[0]) * Integer.parseInt(cuv_2[1]) - Integer.parseInt(cuv_2[0]) * Integer.parseInt(cuv[1]);
        }
        if (N % 2 == 1) {
            line_1 = line_2;
            line_2 = scanner.nextLine();
            cuv_2 = line_2.split(" ");
            cuv = line_1.split(" ");
            sum += Integer.parseInt(cuv[0]) * Integer.parseInt(cuv_2[1]) - Integer.parseInt(cuv_2[0]) * Integer.parseInt(cuv[1]);
        }
        writer.println(sum);
    }
}