#include <bits/stdc++.h>

///#define Nmax 100002

using namespace std;

const int Nmax=100002;

ifstream in("aria.in");
ofstream out ("aria.out");

int n;
double x[Nmax],y[Nmax];

int main()
{
    in>>n;
    for(int i=1;i<=n;i++)
    {
        in>>x[i]>>y[i];
    }
    x[n+1]=x[1];
    y[n+1]=y[1];
    double A=0;
    for(int i=1;i<=n;i++)
    {
        A+=x[i]*y[i+1]-x[i+1]*y[i];
    }
    if(A<0)
        A=-A;
    A/=2;
    out<<fixed<<setprecision(10)<<A;
    out.close();
    return 0;
}