#include <bits/stdc++.h>

using namespace std;

ifstream in("arie.in");
ofstream out("arie.out");
struct poz{double x, y;}a[100002];
int main()
{long long n,x,y,i;
long long ar=0;
in>>n;
for(i=1;i<=n;i++)
{
    in>>a[i].x>>a[i].y;
}
a[n+1]=a[1];
for(i=1;i<=n;i++)
{
    ar+=(a[i].y+a[i+1].y)*(a[i+1].x-a[i].x);
}
ar=abs(ar);
out<<1.0*ar/2<<'\n';
    return 0;
}