#include <iostream>
#include <fstream>
using namespace std;
struct punct
{
    double x,y;
}v[100005];
double sol;
int main()
{
    ifstream in("aria.in");
    ofstream out("aria.out");
    int n,i;
    in>>n;
    for(i=1;i<=n;i++)
        in>>v[i].x>>v[i].y;

    v[n+1].x=v[1].x;
    v[n+1].y=v[1].y;

    for(i=1;i<=n;i++)
        sol+=(v[i].x*v[i+1].y-v[i].y*v[i+1].x);
   if(sol<0)
    out<<sol*(-1)/2.0;
    else
        out<<sol/2.0;


    return 0;
}