#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

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

struct punct
{
    double x,y;
}v[1000005];

int n;
double sum=0;

int main()
{
    fin >> n;
    for(int i=0;i<n;i++)
        fin >> v[i].x >> v[i].y;
         v[n].x = v[0].x;
    v[n].y = v[0].y;
    for(int i=0;i<n;i++)
        sum+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    fout << fixed << setprecision(5) << sum/2;
    return 0;
}