#include<bits/stdc++.h>
using namespace std;
double arie,v[100001],v2[100001];
int n;
int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");
    f>>n;
    for(int i=1; i<=n; i++)
    {
        f>>v[i];
        f>>v2[i];
    }
    arie = 0;
    for(int i=1; i<n; i++)
    {
        arie+=(double)0.5*(v[i]*v2[i+1]-v[i+1]*v2[i]);
    }
    g<<fixed<<setprecision(6)<<arie+(double)0.5*(v[n]*v2[1]-v[1]*v2[n]);
}