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

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

long double x[100005],y[100005];

long double arie=0,suma;

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