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

using namespace std;

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

struct puncte
{
    int x, y;
}p[100005];

int n;

void citire()
{
    fin>>n;
    for(int i=0; i<n; i++)
        fin>>p[i].x>>p[i].y;
}

double arie()
{
    double a=0;
    for(int i=0; i<n-1; i++)
        a+=(p[i].x*p[i+1].y-p[i+1].x*p[i].y)/2;
    return a+(p[n-1].x*p[0].y-p[0].x*p[n-1].y)/2;
}

int main()
{
    citire();
    fout<<setprecision(5)<<fixed<<arie();
    return 0;
}