#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n;
double ar;
struct imp
{
    double a,b;
};
imp v[100002];
int main()
{
    f>>n;
    for (int i=1; i<=n; i++)
    {
        f>>v[i].a>>v[i].b;
    }
    v[n+1]=v[1];
    for (int i=1;  i<=n; i++)
    {
        ar+=(((double)v[i].a*(double)v[i+1].b-(double)v[i+1].a*(double)v[i].b))/2;
    }
    g<<fixed<<setprecision(6)<<ar;
    return 0;
}