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

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

struct Punct{
    double x, y;
} v[100003];

int n;

double Aria( double x1, double y1, double x2, double y2);

int main()
{
    fin >> n;
    for ( int i = 1; i <= n; ++i )
        fin >> v[i].x >> v[i].y;
    v[n + 1].x = v[1].x;
    v[n + 1].y = v[1].y;
    double ANS = 0;
    for ( int i = 2; i <= n + 1; ++i )
        ANS += Aria(v[i - 1].x, v[i - 1].y, v[i].x, v[i].y);
    fout << setprecision(6) << fixed;
    fout << ANS;
    return 0;
}

double Aria( double x1, double y1, double x2, double y2)
{
    double a = x1 * y2 - x2 * y1;
    return a / 2;
}