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

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

const int maxn = 1e5 + 5;
pair <double,double> P[maxn];

int main() {
    ios_base::sync_with_stdio(false);
    int n, i;
    double area = 0;
    fin >> n;
    for (i = 1; i <= n; i++) {
        fin >> P[i].first >> P[i].second;
    }
    P[n + 1] = P[1];
    P[0] = P[n];
    for (i = 1; i <= n; i++) {
        area += P[i].first * (P[i + 1].second - P[i - 1].second);
    }
    fout << fixed << setprecision(5) << area/2.0;
    fin.close();
    fout.close();
    return 0;
}