#include <iostream>
#include <fstream>
#include <vector>
#include <iomanip>
#include <algorithm>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct punct
{
    long double x,y;
};
int n;
punct v[100005];
void citire()
{
    fin>>n;
    for(int i=0; i<n; i++)
    {
        fin>>v[i].x>>v[i].y;
    }
}

void aria()
{   long double s=0;
    for(int i=0; i<n-1; i++)
    {
        s+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y)/2;
    }
     s+=(v[n-1].x*v[0].y-v[0].x*v[n-1].y)/2;
     fout<<setprecision(5)<<fixed;
     fout<<s;
}
int main()
{
    citire();
    aria();
    return 0;
}