#include <fstream>
#include <iomanip>

using namespace std;

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

pair<double, double> pct[100100];

double det(pair<double, double> one, pair<double, double> two){
        return ( one.first * two.second - one.second * two.first ) / 2;
}

int main() {
    int n;
    cin>>n;
    double sum = 0;
    for (int i=1; i<=n; i++){
        cin>>pct[i].first>>pct[i].second;
    }
    pct[0].first  = pct[n].first;
    pct[0].second  = pct[n].second;
    for (int i=1; i<=n; i++){
        sum += det (pct[i - 1], pct[i]);
    }
    cout<<setprecision(5)<<fixed<<sum;
    return 0;
}