import java.util.Scanner;public class newexerc20{ public static void main(String[] args) { Scanner s; int v[][], n, i, area = 0; s = new Scanner(System.in); n = s.nextInt(); v = new int[n][2]; for (i = 0; i < n; i++) { v[i][0] = s.nextInt(); //recebe x v[i][1] = s.nextInt(); //recebe y } for (i = 0; i < n; i++) { if (i == 0) { area = area + v[i][0] * v[i + 1][1] - v[i][0] * v[n - 1][1]; } else if (i == (n - 1)) { area = area + v[i][0] * v[0][1] - v[i][0] * v[i - 1][0]; } else { area = area + v[i][0] * v[i + 1][1] - v[i][0] * v[i - 1][1]; } } System.out.println(area / 2); }}