#include <iostream>
#include <fstream>
#include <math.h>
using namespace std;

ofstream fo("aria.out");
ifstream fi("aria.in");



double calculeazaArie(double a1,double b1,double a2,double b2)
{

    return (a1*b2-a2*b1)/2.0;
}

int main()
{
    double n,x,y,Arie=0,a,b,copiex,copiey;
    fi>>n;
    fi>>a>>b;
    copiex=a;
    copiey=b;
    for(int i=2; i<=n; i++)
    {
        fi>>x>>y;
        Arie=Arie+calculeazaArie(a,b,x,y);
        a=x;
        b=y;
    }
    Arie=Arie-calculeazaArie(copiex,copiey,a,b);

    fo<<Arie;
    return 0;
}