#include<iostream>
#include<iomanip>
#include<fstream>
using namespace std;
int n,i,j,k,m;
long double A;
struct marciuc{
    long double x,y;
};
marciuc v[100005];
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]=v[1];
    A=0.0;
        for(i=1;i<=n;i++)
        A+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
       cout<<setprecision(6)<<fixed<<abs(A/2);
}