#include <bits/stdc++.h>

using namespace std;

struct punct
{
    double x, y;
};

punct v[100005];
double s;
int n;

int main()
{
    freopen ("aria.in", "r", stdin);
	freopen ("aria.out", "w", stdout);
    scanf ("%d", &n);
    for(int i=1; i<=n; i++)
    {
        scanf ("%lf %lf", &v[i].x, &v[i].y);
    }
    v[n+1]=v[1];
    for(int i=1; i<=n; i++)
    {
        s+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    }
    cout<<setprecision(6)<<fixed<<fabs(s/2.0)<<'\n';
    return 0;
}