import java.io.BufferedWriter;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.util.Locale;
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)) {
			scanner.useLocale(Locale.US);
			int nPoints = scanner.nextInt();
			
			float oldxi = scanner.nextFloat();
			float oldyi = scanner.nextFloat();
			float firstxi = oldxi;
			float firstyi = oldyi;
			float sum = 0;
			for(int i=0;i<(nPoints-1);i++) {
				float xi = scanner.nextFloat();
				float yi = scanner.nextFloat();
				sum+= (oldxi*yi-xi*oldyi);
				oldxi = xi;
				oldyi = yi;
			}
			sum+= (oldxi*firstyi - firstxi*oldyi);
			
			bwriter.write(String.valueOf(sum/2.0));
			bwriter.newLine();
		}
	}

}