#include <fstream>

using namespace std;
ifstream cin ("aria.in");
ofstream cout ("aria.out");
#define max 0.0000001
int n;
struct bla
{
    double x,y;
} punct[100010];
void read ()
{
    cin>>n;
    for(int i=1;i<=n;i++)
        cin>>punct[i].x>>punct[i].y;
        punct[n+1]=punct[1];
}
void solve ()
{
    double arie=0;
    for(int i=1;i<=n;i++)
        arie=arie+max+punct[i].x*punct[i+1].y-punct[i+1].x*punct[i].y;
    arie=arie/2;
    cout<<arie+max;
}
int main()
{
    read();
    solve();
    cin.close();
    cout.close();
    return 0;
}