#include <bits/stdc++.h>
#include <iomanip>
using namespace std;
//ifstream fin ("aria.in");
//ofstream fout("aria.out");
struct ceva
{
    double x,y;
}v[100010];
int n,i;
long double ans;
int main()
{
    freopen ("aria.in", "r", stdin);
    freopen ("aria.out", "w", stdout);
    scanf ("%d", &n);
    for(i=1;i<=n;i++)
        scanf ("%lf %lf", &v[i].x, &v[i].y);
    v[n+1]=v[1];
    for(i=1;i<=n;i++)
        ans=ans+(v[i].x*v[i+1].y)-(v[i+1].x*v[i].y);
    cout<<fixed<<setprecision(6)<<fabs(ans/2.0);
    return 0;
}