#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n;
double x[100001], y[100001], sum;
int main() {
    fin >> n;
    for (int i = 0; i < n; i++)
        fin >> x[i] >> y[i];
    for (int i = 0; i < n; i++) {
        int j = (i + 1) % n;
        sum += x[i] * y[j] - x[j] * y[i];
    }
    sum = abs(sum) / 2.0;
    fout << fixed << setprecision(6) << sum;
    return 0;
}