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

struct pct
{
    long double x,y;
}Pol[100005];

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

int n;
long double A=0;

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


double Trunghi(pct A,pct B)
{
    return ((A.x*B.y)-(B.x*A.y))*0.5;
}


void arie()
{
    for(int i=0;i<n-1;i++)
        A+=Trunghi(Pol[i],Pol[i+1]);

    A+=Trunghi(Pol[n-1],Pol[0]);

    e<<setprecision(5)<<A;
}

int main()
{
    //freopen("aria.in","r",stdin);
    //freopen("aria.out","w",stdout);
    citire();
    arie();
    return 0;
}