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

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

struct puncts
{
    float x, y;
};
int n;
float triunghi(puncts a, puncts b)
{
    return a.x*b.y-a.y*b.x;
}
void prelucrare()
{
    f>>n;
    puncts p1,copiep1,p;

    if (n <= 2)
    {
        g << 0;
    }

    f>>p1.x>>p1.y;
    copiep1.x=p1.x;
    copiep1.y=p1.y;
    float S=0;

    for(int i=2; i<=n; i++)
    {

        f>>p.x>>p.y;
        S+=triunghi(p1, p);
        p1=p;
    }

    S+=triunghi(p, copiep1);

    g<< fixed << setprecision(5) <<abs(S)/2;
}
int main()
{
    prelucrare();
    return 0;
}