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

using namespace std;

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

struct DB
{
    int x, y;
};

DB a[NMax];
int aria;
double aria1;
int n;
void Citire()
{
    int i;

    fin >> n;

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

void Rezolva()
{
    int i;

    for(i = 1; i <= n; i++)
        aria += (a[i].x * a[i + 1].y)
        - (a[i + 1].x * a[i].y);
    ///cout << aria;
    aria += (a[n].x * a[1].y)
        - (a[1].x * a[n].y);
    if(aria % 2 == 0)
        fout << aria / 2 << "\n";
    else
    {
        aria1 = aria / 2;
        fout << fixed << setprecision(5) << aria1 << "\n";
    }
}
int main()
{
    Citire();
    Rezolva();
    return 0;
}