#include <fstream>
#include <iomanip>
#include <cmath>

using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");

double x[100005], y[100005];

int main()
{
    long double arie = 0;
    int n, primx, primy, prevx, prevy;
    cin>>n;
    cin>>primx>>primy;
    prevx = primx;
    prevy = primy;
    for(int i = 2; i <= n; ++i)
    {
        int x, y;
        cin>>x>>y;
        arie +=prevx * y - x * prevy;
        prevx = x;
        prevy = y;
    }
    arie += prevx * primy - primx * prevy;
    arie = abs(arie);
    arie /= 2;
    cout<<setprecision(5)<<fixed<<arie;
    return 0;
}