#include <iostream>
#include <iomanip>
#include <math.h>
#include <cstdio>

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

struct punct
{
    double linie, coloana;
} v[100005];

int n;


int main()
{
    freopen ("aria.in", "r", stdin);
    freopen ("aria.out", "w", stdout);
    scanf("%d", &n);
    for(int i=1; i<=n; i++)
        scanf("%lf %lf", &v[i].coloana, &v[i].linie);
    double rezultat=0.0;
    v[n+1]=v[1];
    for(int i=1; i<=n; i++)
    {
        rezultat+=(v[i].coloana*v[i+1].linie - v[i+1].coloana*v[i].linie);
    }
    cout<<setprecision(6)<<fixed<<fabs(rezultat/2.0);
    return 0;
}