#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
const int NMAX=100005;
int i, n;
double s;
struct points
{
    double x,y;
} v[NMAX];
int main()
{
    ifstream cin("aria.in");
    ofstream cout("aria.out");
    cin>>n;
    for(i=1; i<=n; i++)
        cin>>v[i].x>>v[i].y;
    v[n+1].x=v[1].x;
    v[n+1].y=v[1].y;
    for(i=1; i<=n; i++)
        s=s+(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    s/=2;
    cout<<setprecision(5)<<fixed<<s<<'\n';
    return 0;
}