#include <bits/stdc++.h>
#define int long long
using namespace std;
struct cows
{
    int x,y;
};
cows v[100005];

signed main()
{
    ifstream cin("aria.in");
     ofstream cout("aria.out");
    int n;
    cin>>n;
    for(int i=1;i<=n;i++)
    {
        cin>>v[i].x>>v[i].y;
    }
    int ans=v[1].x*v[n].y-v[n].x*v[1].y;
    for(int i=2;i<=n;i++)
    {
        ans+=v[i].x*v[i-1].y-v[i-1].x*v[i].y;
    }
   /// ans=abs(ans);
    if(ans%2==1)
        cout<<ans/2<<".5";
    else
        cout<<ans/2;

    return 0;
}