#include <fstream>
#include <iomanip>
#define DIM 100010
 
using namespace std;
 
pair<double, double> v[DIM];
 
double sol;
int n, i;
 
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 main() {
    ifstream fin("aria.in");
    ofstream fout("aria.out");
 
    fin>>n;
    for (i=1;i<=n;i++) {
        fin>>v[i].first>>v[i].second;
    }
    v[0] = v[n];
 
    for (i=1;i<n;i++) {
        sol += aria(v[0], v[i], v[i+1]);
    }
 
    fout<<setprecision(7)<<fixed<<sol/2;
    return 0;
}