#include <bits/stdc++.h>
#define MAXN 100005

using namespace std;

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

int n;
double sol, x, y, prevx, prevy, ultimx, ultimy;

int main()
{
    in >> n;
    in >> prevx >> prevy;
    ultimx=prevx;
    ultimy=prevy;
    for (int i=2; i<=n; i++)
    {
        in >> x >> y;
        sol+=prevx*y-x*prevy;
        prevx=x;
        prevy=y;
    }
    sol+=prevx*ultimy-ultimx*prevy;
    sol/=2;
    out << fixed << setprecision(6) << sol;
    return 0;
}