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

ifstream fin("aria.in");
ofstream fout("aria.out");
double x[100010],y[100010];
long double ras;
    int i,n;
int main()
{


    fin>>n;
    for(i=1;i<=n;i++)
        fin>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(i=1;i<=n;i++)
        ras+=(x[i]*y[i+1]-y[i]*x[i+1]);
    if(ras<0)ras=ras/-2.0;
    else ras=ras/2.0;
    fout<<setprecision(5)<<ras<<'\n';
}