import java.io.IOException;
import java.io.InputStream;
import java.text.DecimalFormat;
import java.util.NoSuchElementException;

public class Main {
	private static FastScanner sc = new FastScanner();
	private static double ERROR = 0.0000001;
	
	public static void main(String[] args) {
		int n = sc.nextInt();
		Point[] p = new Point[n];
		for(int i=0; i<n; i++) {
			p[i] = new Point(sc.nextDouble(), sc.nextDouble());
		}
		
		double sum_area = 0;
		for(int i=0; i<n; i++) {
			Point origin = new Point(0, 0);
			if(checkCounterClockwise(p[i], p[(i+1)%n], origin)) {
				sum_area += area(p[i], p[(i+1)%n], origin);
			} else {
				sum_area -= area(p[i], p[(i+1)%n], origin);
			}
		}
		
		DecimalFormat df = new DecimalFormat("0.0##");
		System.out.println(df.format(Math.abs(sum_area)));
	}

	static double distance(Point p1, Point p2) {
		return Math.sqrt((p2.x-p1.x)*(p2.x-p1.x) + (p2.y-p1.y)*(p2.y-p1.y));
	}
	
	static double area(Point p1, Point p2, Point p3) {
		double a = distance(p1, p2);
		double b = distance(p2, p3);
		double c = distance(p3, p1);
		double s = (a + b + c) / 2;
		if(s < Math.max(a, Math.max(b, c))+ERROR && s+ERROR > Math.max(a, Math.max(b, c))) {
			return 0;
		}
		return Math.sqrt(s * (s - a) * (s - b) * (s - c));
	}
	
	static class Point {
		double x;
		double y;
		Point(double x, double y) {
			this.x = x;
			this.y = y;
		}
	}
	
	static boolean checkCounterClockwise(Point p1, Point p2, Point p3) {
		double a = (p2.y - p1.y) / (p2.x - p1.x);
		double b = p2.y - a * p2.x;
		if(p1.x == p2.x) {
			if((p1.y < p2.y && p3.x < p1.x) || (p1.y > p2.y && p3.x > p1.x)) {
				return true;
			} else {
				return false;
			}	
			
		} else {
			if((p1.x < p2.x && p3.y - (a * p3.x + b) > 0) || (p1.x > p2.x && p3.y - (a * p3.x + b) < 0)) {
				return true;
			} else {
				return false;
			}				
		}
	}
	
	static class FastScanner {
        private final InputStream in = System.in;
        private final byte[] buffer = new byte[1024];
        private int ptr = 0;
        private int buflen = 0;
        private boolean hasNextByte() {
            if(ptr < buflen) {
                return true;
            } else {
                ptr = 0;
                try {
                    buflen = in.read(buffer);
                } catch(IOException e) {
                    e.printStackTrace();
                }
                if(buflen <= 0) {
                    return false;
                }
            }
            return true;
        }
        private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
        private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
        private void skipUnprintable() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;}
        public boolean hasNext() { skipUnprintable(); return hasNextByte();}
        public String next() {
            if (!hasNext()) throw new NoSuchElementException();
            StringBuilder sb = new StringBuilder();
            int b = readByte();
            while(isPrintableChar(b)) {
                sb.appendCodePoint(b);
                b = readByte();
            }
            return sb.toString();
        }
        public long nextLong() {
            return Long.parseLong(next());
        }
        public int nextInt(){
            return Integer.parseInt(next());
        }
        public double nextDouble(){
            return Double.parseDouble(next());
        }
    }
}

Measured memory_bytes: 26108