import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Stream;

public class Main {
    public static void main(String[] args) {
        final BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        final BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
        try {
            int T = 1;
            for (int i = 0; i < T; ++i) {
                IRunner runner = new Runner(br, bw);
                runner.run();
                runner.flush();
            }
            br.close();
            bw.close();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

class Solution {
    final int N;
    final long[][] points;

    public Solution(int N, long[][] points) {
        this.N = N;
        this.points = points;
    }

    public String solution() {
        // use Shoelace Formula
        long sum = 0;
        for (int i = 0; i < N; ++i) {
            var n1 = points[i][0] * points[i + 1][1];
            var n2 = points[i][1] * points[i + 1][0];
            sum += n1 - n2;
        }
        return String.format("%.1f", Math.abs(sum) / 2.0);
    }
}

class Runner implements IRunner {
    final IReader reader;
    final BufferedWriter bw;
    final StringBuilder sb = new StringBuilder();

    final int N;
    final long[][] points;

    Runner(BufferedReader br, BufferedWriter bw) {
        this.reader = new Reader(br);
        this.bw = bw;
        try {
            this.N = reader.readInts()[0];
            this.points = new long[N + 1][];
            for (int i = 0; i < N; ++i) {
                String[] input = reader.line().split(" ");
                points[i] = new long[] {
                        Long.parseLong(input[0]),
                        Long.parseLong(input[1])
                };
            }
            points[N] = points[0];

            sb.ensureCapacity(20);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    @Override
    public void flush() {
        try {
            bw.write(sb.toString());
            bw.flush();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    @Override
    public void run() throws IOException {
        var sol = new Solution(N, points);
        var res = sol.solution();
        sb.append(res).append('\n');
    }
}

class Reader implements IReader {
    private BufferedReader br;

    public Reader(BufferedReader br) {
        this.br = br;
    }

    @Override
    public void skipLine() throws IOException {
        br.readLine();
    }

    @Override
    public Stream<String> lines() throws IOException {
        return br.lines();
    }

    @Override
    public String line() throws IOException {
        return br.readLine();
    }

    @Override
    public int[] readInts() throws IOException {
        String[] tokens = br.readLine().split(" ");
        int[] ints = new int[tokens.length];
        for (int i = 0; i < tokens.length; ++i) {
            ints[i] = Integer.parseInt(tokens[i]);
        }
        return ints;
    }

    @Override
    public List<Integer> readIntegers() throws IOException {
        String[] tokens = br.readLine().split(" ");
        var res = new ArrayList<Integer>(tokens.length);
        for (String token : tokens) {
            res.add(Integer.valueOf(token));
        }
        return res;
    }
}

interface IRunner {
    void run() throws IOException;

    void flush();
}

interface IReader {
    void skipLine() throws IOException;

    Stream<String> lines() throws IOException;

    String line() throws IOException;

    List<Integer> readIntegers() throws IOException;

    int[] readInts() throws IOException;
}