#include <bits/stdc++.h>
using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");
const short Nmax = 1001;
struct P
{
   double x , y;
};
P a[Nmax];
int n;
inline double A(P A , P B , P C)
{
    double ar;
    ar = abs(A . x * B .y + A . y * C . x +
             B . x * C . y - B . y * C . x - B . x * A . y - A . x * C . y);
    ar = (double) ar / 2;
    return ar;
}
int main()
{
    double ar = 0;
    fin >> n;
    for(int i = 1 ; i <= n ; i++)
        fin >> a[i] . x >> a[i] . y;
    for(int i = 2 ; i < n ; i++)
        ar += A(a[1] , a[i] , a[i + 1]);
    fout << fixed << setprecision(6) << ar << "\n";
    fin.close();
    fout.close();
    return 0;
}