#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
double x[100014],y[100014],A;
int main()
{
    fstream f,g;
    f.open("aria.in",ios::in);
    g.open("aria.out",ios::out);
    int i,n;
    f>>n;
    for (i=1;i<=n;i++)
        f>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];
    for (i=1;i<=n;i++)
        A=A+(x[i]*y[i+1]-x[i+1]*y[i])/2;
    g<<fixed<<setprecision(5)<<A;
}