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#include <fstream>
#include <iomanip>
using namespace std;
#define double long double
double x[100005];
double y[100005];
ifstream cin("aria.in");
ofstream cout("aria.out");
signed main()
{
    int n;
    double ans=0;
    cin>>n;
    for(int i=1;i<=n;i++)
    {
       cin>>x[i]>>y[i]; 
    }
    for(int i=2;i<=n;i++)
    {
     ans+=x[i-1]*y[i]-y[i-1]*x[i];   
    }
    ans+=x[n]*y[1]-y[n]*x[1];
    ans=ans/2.0;
    ans=abs(ans);
    cout<< setprecision(5) << fixed << ans;
    return 0;
}