#include <iostream>
#include <fstream>
#include <iomanip>

#define NMAX 100005

using namespace std;

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

int n;
long double x[NMAX], y[NMAX];
long double aria;

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

    x[n] = x[0]; y[n] = y[0];
    for (int i = 0; i < n; i++)
        aria += x[i] * y[i+1] - x[i+1] * y[i];

    fout << fixed << setprecision(5) << aria / 2.0;
    return 0;
}