#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
int n;
struct punct
{
    long double x,y;
} v[100005];
long double sol;
void read()
{
    cin>>n;
    for(int i=1; i<=n; i++)
        cin>>v[i].x>>v[i].y;
}
void solve()
{
    v[n+1]=v[1];
    for(int i=1; i<=n; i++)
        sol+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    sol/=2;
    if(sol<0)
        sol*=-1;
    cout<<fixed<<setprecision(6)<<sol;
}
int main()
{
    read();
    solve();
    return 0;
}