#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;

int main() {
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    
    int n;
    fin >> n;
    
    double x[100000], y[100000];
    for (int i = 0; i < n; i++) {
        fin >> x[i] >> y[i];
    }
    
    double area = 0.0;
    for (int i = 0; i < n; i++) {
        int j = (i + 1) % n;
        area += x[i] * y[j] - x[j] * y[i];
    }
    
    area = abs(area) / 2.0;
    
    fout << fixed << setprecision(5) << area;
    
    fin.close();
    fout.close();
    
    return 0;
}