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

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

int n;
struct pct{
    double x, y;
}initial, pct1, pct2;

long double ariefinala, arie;

int main()
{
    //freopen("aria.out","w",stdout);
    f >> n;
    f >> initial.x >> initial.y;
    f >> pct1.x >> pct1.y;
    for (int i=1; i<n-1; i++)
    {
        f >> pct2.x >>pct2.y;
        arie=0;
        arie=initial.x*pct1.y+pct1.x*pct2.y+pct2.x*initial.y-initial.x*pct2.y-initial.y*pct1.x-pct1.y*pct2.x;
        ariefinala+=arie;
        pct1.x=pct2.x;
        pct1.y=pct2.y;
    }
    if (ariefinala<0)
        ariefinala=-(ariefinala);
    g << setprecision(6)<<fixed<<ariefinala/2;
    return 0;
}