#include<bits/stdc++.h>
using namespace std;
ifstream fin ("aria.in");
ofstream fout("aria.out");

struct pct
{
    double x,y;
}v[100002];

int main ()
{
    int n,i;
    double ans=0;
    fin>>n;
    for (i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for (i=1;i<=n;i++)
        ans=ans + (double) (v[i].x * v[i+1].y) - (double)(v[i].y*v[i+1].x);
    ans=(double)ans/2;
    fout<<fixed<<setprecision(6)<<ans<<'\n';
    return 0;
}