#include <bits/stdc++.h>

using namespace std;

int main()
{
    int n, x1, y1, xi, yi, xj, yj;
    double arie = 0;
    ifstream fin ("aria.in");
    ofstream fout ("aria.out");
    fin >> n;
    fin >> x1 >> y1;
    xi = x1;
    yi = y1;
    for (int i = 1; i<n; ++i)
    {
        fin >> xj >> yj;
        arie += (xi*yj-xj*yi);
        xi = xj;
        yi = yj;
    }
    arie += (xi*y1-x1*yi);
    fout << arie/2.0;
    return 0;
}