#include <fstream>

using namespace std;

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

struct punct
{
    long double x, y;
};

int n;
long double a;
punct v[100010];

int main()
{
    int i;
    fin >> n;
    for (i = 1; i <= n; i++)
        fin >> v[i].x >> v[i].y;
    v[n + 1] = v[1];
    for (i = 1; i <= n; i++)
        a += v[i].x * v[i + 1].y - v[i + 1].x * v[i].y;
    a /= 2.0;
    fout << a << '\n';
    fout.close();
    return 0;
}