#include <iostream>
#include <fstream>

using namespace std;

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

struct punct{
    double x, y;
}nod[100001];

double sum=0;
int n;

double aria(punct a, punct b, punct c)
{

    double s=0;
    s=((a.x*b.y)+(a.y*c.x)+(b.x*c.y))-((c.x*b.y)+(a.x*c.y)+(a.y*b.x));
    if(s<0)
        s=s*(-1);

    return double(s)/2;
}

int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
    {
        fin>>nod[i].x>>nod[i].y;
    }
    for(int i=2;i<n;i++)
        sum+=aria(nod[1],nod[i],nod[i+1]);
    fout<<sum;
    return 0;
}