#include<fstream>
#include<cmath>
#include<iostream>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n;
double x[100005],y[100005],mx,my,a;
double arie(int a,int b,int c)
{
    mx=x[c];
    my=y[c];
    return abs(x[a]*y[b]+x[b]*my+mx*y[a]-mx*y[b]-my*x[a]-y[a]*x[b]);
}
int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
    {
        fin>>x[i]>>y[i];
        mx+=x[i];
        my+=y[i];
    }
    mx/=n;
    my/=n;
   // cout<<mx<<' '<<my;
    n++;
    x[n]=x[1];
    y[n]=y[1];
    for(int i=3;i<=n;i+=2)
    {
        a+=arie(i,i-1,i-2)/2;
        //cout<<arie(i,i-1,i-2)/2<<'\n';
    }
    fout<<a;
}