#include <iostream>
#include <fstream>
#include <math.h>
#include <iomanip>
using namespace std;

int main() 
{
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    double x1,y1,x2,y2,a,b;
    double T=0;
    int N,i;
    fin>>N;
    fin>>x1>>y1;
    a=x1;
    b=y1;
    for(i=2;i<=N;i++)
    {
        fin>>x2>>y2;
        T=T+(x1*y2-x2*y1);
        x1=x2;
        y1=y2;
    }
    T=T+(x2*b-y2*a);
    fout<<std::fixed<<std::setprecision(5);
    fout<<T/2;
    fin.close();
    fout.close();
    return 0;
}