// cu trapeze

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

using namespace std;

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

#define lim 100010
struct hei{double x,y;};
hei ini[lim];
int n,i;
long double rez;

int modul(long double x)
{
    if(x<0)
        return -x;
    return x;
}


int main()
{
    fin>>n;
    for(i=1; i<=n; i++)
        fin>>ini[i].x>>ini[i].y;
    ini[n+1]=ini[1];

    rez=0;

    for(i=1; i<=n; i++)
        rez+=( (ini[i].y+ini[i+1].y) * (ini[i+1].x-ini[i].x) /2.0 );

    fout<<setprecision(5)<<fixed<<modul(rez);

    fin.close();
    fout.close();
    return 0;
}