#include <bits/stdc++.h>

using namespace std;

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

double aria (pair <double, double> a, pair <double, double> b, pair <double, double> c){
    return (b.first - a.first) * (c.second - a.second) - (c.first - a.first) * (b.second - a.second);
}

int n;

double arie;

pair <double, double> v[100005];

int main(){
    fin >> n;
    for (int i=1; i<=n; i++){
        fin >> v[i].first >> v[i].second;
    }
    v[0] = v[n];
    for (int i=0; i<n; i++){
        arie += aria (v[0], v[i], v[i+1])/2.0;
    }
    fout << fixed << setprecision(6) << arie;
    return 0;
}