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

using namespace std;

const int MAX_N = 100000;
double x[MAX_N], y[MAX_N];

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

    int n;
    fin >> n;

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

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

	fout << fabs( s / 2 );

	fin.close();
	fout.close();

    return 0;
}