1.5-4

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int l = sc.nextInt();
        int[][] arr = new int[l][];
        for(int i = 0; i<l;++i){
            int[] a = new int[2];
            for (int j = 0;j<2;++j){
                a[j] = sc.nextInt();
            }
            arr[i] = a;
        }
        int s = arr[0][1]*arr[arr.length-1][0]-arr[0][0]*arr[arr.length-1][1];
        for (int i = 0;i<l-1;++i){
            s+=arr[i][0]*arr[i+1][1];
            s-=arr[i+1][0]*arr[i][1];
        }
        double sfinal = Math.abs(s);
        System.out.println(sfinal/2);
    }
}