#include <iostream>
#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct triunghi
{
    double x,y;
};
triunghi a[100001];
int n,s;
int main()
{
    int i;
    f>>n;
    for(i=1;i<=n;i++)
    {
        f>>a[i].x>>a[i].y;
    }
    a[n+1]=a[1];
    for(i=1;i<=n;i++)
    {
        s=s+(a[i].x*a[i+1].y-a[i+1].x*a[i].y);
    }
    s=s/2;
    g<<fixed<<setprecision(5)<<abs(s);
    return 0;
}