#include <bits/stdc++.h>
using namespace std;

int n;
struct punct{
  long  double x,y;
}a[100010],b;
long double ans;
int main()
{
    ifstream cin("aria.in");
    ofstream cout("aria.out");

    cin>>n;
    if(n<=2) cout<<0;
    else
    {
        cin>>a[0].x>>a[0].y;
        cin>>a[1].x>>a[1].y;
        b=a[1];
        for(int i=2;i<n;i++)
        {
            cin>>a[i].x>>a[i].y;
            ans+=abs(a[0].x*b.y+b.x*a[i].y+a[i].x*a[0].y-b.y*a[i].x-a[i].y*a[0].x-a[0].y*b.x)/2;
            b=a[i];
        }
        cout<<setprecision(5)<<fixed<<ans;
    }

    return 0;
}