#include<fstream>
#include<cmath>
#include<iomanip>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
int n;
double sol;
struct
{
    double x,y;
}v[100005],pct;
int main()
{
    cin>>n;
    for(int i=1;i<=n;i++)
        cin>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for(int i=1;i<=n;i++)
        sol+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y)/2.0;
    cout<<setprecision(6)<<fixed<<fabs(sol);
    return 0;
}