#include <iostream>
#include <fstream>
#include <cmath>
using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");
int main()
{
    int i,n,x[100001],y[100001];
    double ras=0;
    fin>>n;
    for(i=0;i<n;i++)
        fin>>x[i]>>y[i];
    x[n]=x[0];
    y[n]=y[0];
    for(i=0;i<n;i++)
        ras+=x[i]*y[i+1]-y[i]*x[i+1];
    fout<<fabs(ras/2.0)<<'\n';
}