#include <iostream>
#include <cmath>
#include <vector>
#include <string>
#include <set>
#include <algorithm>
#include <utility>
#include <iomanip>
using namespace std;

void task4() {
    int n;
    cin >> n;
    vector <int> x_coordinates;
    vector <int> y_coordinates;
    for (int i = 0; i < n; i++) {
        int x, y;
        cin >> x >> y;
        x_coordinates.push_back(x);
        y_coordinates.push_back(y);
    }
    int first_area = 0, second_area = 0;
    double area;
    x_coordinates.push_back(x_coordinates[0]);
    y_coordinates.push_back(y_coordinates[0]);
    for (int i = 1; i < y_coordinates.size(); i++) {
        first_area += x_coordinates[i - 1] * y_coordinates[i];
    }
    for (int i = 1; i < x_coordinates.size(); i++) {
        second_area += x_coordinates[i] * y_coordinates[i - 1];
    }
    area = (first_area - second_area) / 2;
    cout << showpoint << setprecision(2) << area;
}

//struct time {
//  int hours;
//  int minutes;
//  int seconds;
//  vector <int> marks;
//};
//
//bool time_compare(const time& first_time, const time& second_time) {
//  if (first_time.hours != second_time.hours) {
//      return first_time.hours < second_time.hours;
//  }
//  else {
//      if (first_time.minutes != second_time.minutes) {
//          return first_time.minutes < second_time.minutes;
//      }
//      else {
//          if (first_time.seconds != second_time.seconds) {
//              return first_time.seconds < second_time.seconds;
//          }
//      }
//  }
//}
//
//void task3() {
//  int n;
//  cin >> n;
//  vector <time> time_moments(n);
//  for (int i = 0; i < n; i++) {
//      time current_time;
//      int current_hours, current_minutes, current_seconds;
//      cin >> current_hours >> current_minutes >> current_seconds;
//      current_time.hours = current_hours;
//      current_time.minutes = current_minutes;
//      current_time.seconds = current_seconds;
//      time_moments[i] = current_time;
//  }
//  stable_sort(time_moments.begin(), time_moments.end(), time_compare);
//  for (auto current_time : time_moments) {
//      cout << current_time.hours << " " << current_time.minutes << " " << current_time.seconds << endl;
//  }
//}

void task2() {
    string number;
    cin >> number;
    set <string> current_number;
    for (int i = 0; i < number.size(); i++) {

    }
}

void task1() {
    int n, members_number = 0;
    cin >> n;
    vector <int> members;
    for (int i = 0; i < n; i++) {
        int current_team;
        cin >> current_team;
        members.push_back(current_team);
        members_number += current_team;
    }
    int new_members;
    cin >> new_members;
    members_number += new_members;
    bool equal_teams = false;
    if (members_number % n == 0) {
        for (auto team : members) {
            if (team == members_number / n) {
                equal_teams = true;
                cout << "NO";
            }
        }
        if (!equal_teams) {
            cout << members_number / n;
        }
    }
    else {
        cout << "NO";
    }
}

int main() {
    //task1();
    //task2();
    //task3();
    task4();

    system("PAUSE");
    return 0;
}