#include <fstream>
#include <iomanip>
#define xx first
#define yy second

using namespace std;

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

long long n,i;
pair<double, double> v[100005];

double det(pair<double, double> a, pair<double, double> b, pair<double, double> c)
{ return (b.xx-a.xx)*(c.yy-a.yy)-(c.xx-a.xx)*(b.yy-a.yy); }

int main()
{
    fin >> n;
    for (i=1; i<=n; i++)
        fin >> v[i].xx >> v[i].yy;
    v[n+1] = v[1]; double sol = 0;
    for (i=1; i<=n; i++)
        sol += det(v[0], v[i], v[i+1]);
    fout << setprecision(5) << fixed << sol/2.0;
    return 0;
}