#include <iostream>
#include <fstream>
#include <iomanip>
#define nmx 100005
using namespace std;
int n;
long double x[nmx],y[nmx],rsp;
int main()
{
    ifstream f ("aria.in");
    ofstream g ("aria.out");
    f>>n;
    for (int i=1;i<=n;i++)
        f>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];
    g<<fixed<<setprecision(5);
    for (int i=1;i<=n;i++)
        rsp+=x[i]*y[i+1]-x[i+1]*y[i];
    rsp/=2;
    g<<rsp;
}