#include <bits/stdc++.h>
using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

const int N = 100005;
int n;
double s;
pair<double, double> v[N];

int main()
{
    fin>>n;
    for(int i=1; i<=n; i++)
        fin>>v[i].first>>v[i].second;
    v[n+1]=v[1];
    for(int i=1; i<=n; i++)
    {
        s+=v[i].first*v[i+1].second-v[i+1].first*v[i].second;
    }
   s/=2;
   fout<<setprecision(5)<<fixed<<s;
    return 0;
}