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

ifstream in("aria.in");
ofstream out("aria.out");
const int NMax = 100000;

int n;
double x[NMax+5], y[NMax+5];

void Solve() {
    x[n] = x[0];
    y[n] = y[0];
    double sol=0;
    for (int i = 0; i < n; i++)
        sol += x[i] * y[i+1] - y[i] * x[i+1];

    out << fixed << setprecision(6) << sol / 2;
}

int main() {
    in >> n;
    for (int i = 0; i < n; i++)
        in >> x[i] >> y[i];

    Solve();

    return 0;
}