#include <bits/stdc++.h>
#define nmax 100007

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

int n;
int x[nmax], y[nmax];
long double rasp;

void Citire()
{
    int i;
    fin >> n;
    for (i = 1; i <= n; i++)
        fin >> x[i] >> y[i];
    x[++n] = x[1];
    y[n] = y[1];
}

void Rezolvare()
{
    int i;
    for (i = 2; i <= n; i++)
        rasp += (x[i-1] * y[i] - x[i] * y[i-1]);
    fout << fixed << setprecision(7) << rasp / 2.0 << "\n";
}

int main()
{
    Citire();
    Rezolvare();

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