#include <iostream>
#include <fstream>
#include <deque>
#include <vector>
#include <list>
#include <iterator>
#include <queue>
#include <algorithm>
#include <tuple>

using namespace std;

ifstream in ("aria.in");
ofstream out ("aria.out");



int main(){

    int N;
    in >> N;

    vector<pair<double, double>> puncte(N);
    for(auto &punct : puncte){
        in >> punct.first >> punct.second;
    }
    puncte.push_back(puncte[0]);

    double sum = 0;

    for(int i = 0; i < N; i++){
        sum += puncte[i].first * puncte[i + 1].second - puncte[i+1].first * puncte[i].second;
    }

    sum /= 2;

    out << (sum < 0 ? -sum : sum);

    return 0;
}