#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

double modul(double x) {
    if (x < 0)
        return (-x);
    return x;
}

int main() {
    
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    
    int N;
    double x1, y1, x2, y2, sum;

    fin >> N;
    
    fin >> x1 >> y1;
    
    for (int i = 2; i <= N; i++) {
        fin >> x2 >> y2;
        sum += (modul(y1) + modul(y2))*(x2 - x1);
    }
    
    sum = modul(sum);
    
    fout << fixed << setprecision(5) << sum / 2 << "\n";
    
    fin.close();
    fout.close();
    
    return 0;
}