#include <fstream>
#define x first
#define y second

using namespace std;

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

typedef pair<double, double> Punct;

Punct p[100001];

int main()
{
    int n;
    fin>>n;
    for(int i=1;i<=n;i++)
    {
        fin>>p[i].x;
        fin>>p[i].y;
    }
    p[0] = p[n];
    double arie = 0;
    for(int i=0;i<n;i++)
        arie += (p[i].x * p[i+1].y - p[i+1].x*p[i].y)/2;
    fout<<arie;

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