#include <bits/stdc++.h>

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct pucnte
{
   double  x,y;
}v[100010];
int n,i;
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
    {
       f>>v[i].x>>v[i].y ;
    }
    v[n+1]=v[1];
    double arie=0;
    for(i=1;i<=n;i++)
    {
        arie+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y)/2;
    }
    g<<arie<<'\n';
    return 0;
}