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

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

const int MAX = 100010;
struct Punct{
    int x, y;
};
Punct a[MAX];
double s;
int N;

int main()
{
    int i, j;

    fin >> N;
    for ( i = 1; i <= N; i++ )
        fin >> a[i].x >> a[i].y;

    a[N + 1] = a[1];

    for ( i = 1; i <= N; i++ )
        s += a[i].x * a[i + 1].y - a[i].y * a[i + 1].x;

    fout << fabs( s / 2 );

    fin.close();
    fout.close();
    return 0;
}