#include <fstream>
#include <algorithm>
#include <iomanip>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
int n;
pair<long double,long double>v[100005];
long double det(long double x1,long double y1,long double x2,long double y2,long double x3,long double y3)
{
    return (x2-x1)*(y3-y1)-(y2-y1)*(x3-x1);
}
long double sol;
long double modul(long double a)
{
    if(a>=0)
        return a;
    else
        return -a;
}
int main()
{
    cin>>n;
    for(int i=1;i<=n;i++)
        cin>>v[i].first>>v[i].second;
    v[0]=v[n];
    for(int i=0;i<n;i++)
        sol+=det(0,0,v[i].first,v[i].second,v[i+1].first,v[i+1].second);
    cout<<fixed<<setprecision(7)<<modul(sol)/2.0;
    return 0;
}