#include <bits/stdc++.h>

using namespace std;

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

struct point
{
  double x,y;
};
point a[100005];
int n;
int main()
{
    int i;
    double sol=0;
    fin >> n;
    for (i=1; i<=n; i++)
    fin >> a[i].x >> a[i].y;
    a[n+1]=a[1];
    for (i=1; i<=n; i++)
        sol+=(a[i].x * a[i + 1].y - a[i + 1].x * a[i].y);
        sol/=2;
    sol=abs(sol);
    fout << setprecision(9) << fixed << sol;


    return 0;
}