#include <fstream>

using namespace std;

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

int n, i;
double x[100007], y[100007], a;

int main()
{
    fin >> n;

    for(i = 1; i <= n; i++)
        fin >> x[i] >> y[i];

    x[n+1] = x[1];
    y[n+1] = y[1];

    for(i = 1; i <= n; i++)
    {
        a += x[i]*y[i+1] - x[i+1]*y[i];
    }

    fout << a/2.0;

}