#include <iostream>
#include <fstream>

using namespace std;

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

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

pair<double, double> v[100003];

int main() {

    int n;
    in >> n;

    for(int i = 1; i <= n; i++)
        in >> v[i].first >> v[i].second;

    double tot = 0;

    for(int i = 1; i < n; i++)
        tot += cross_product(v[i-1], v[i]);

    out << tot;

    return 0;
}