import java.io.BufferedWriter;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.util.Scanner;

class Main {

	public static void main(String[] args) throws FileNotFoundException, IOException {
		try(FileReader freader = new FileReader("aria.in");
			Scanner scanner = new Scanner(freader);
			FileWriter fwriter = new FileWriter("aria.out");
			BufferedWriter bwriter = new BufferedWriter(fwriter)) {
			int nPoints = scanner.nextInt();
			int [] x = new int[nPoints];
			int [] y = new int[nPoints];
			for(int i=0;i<nPoints;i++) {
				x[i] = scanner.nextInt();
				y[i] = scanner.nextInt();
			}
			int sum = 0;
			int i;
			for(i=0;i<nPoints-1;i++) {
				sum+= (x[i]*y[i+1]-x[i+1]*y[i]);
			}
			sum+= (x[i]*y[0]-x[0]*y[i]);
			bwriter.write(String.valueOf((1/2.0) * sum));
			bwriter.newLine();
		}
	}

}