import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        int x0 = sc.nextInt();
        int y0 = sc.nextInt();
        int x1 = sc.nextInt();
        int y1 = sc.nextInt();
        long space = 0;
        for (int i = 2; i < n; i++) {
            int x = sc.nextInt();
            int y = sc.nextInt();
            space += Math.abs((x - x0) * (y1 - y0) - (x1 - x0) * (y - y0));
            x1 = x;
            y1 = y;
        }
        System.out.println(space);
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public String next() throws Exception {
        if (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}