#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n,i;
long double aria;
struct punct
{
    int x, y;
}v[100005];
int main()
{
    fin>>n;
    for(i=1;i<=n;i++)
    fin>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for(i=1;i<n-1;i++)
        aria+=v[i].x*v[i+1].y+v[i+1].x*v[i+2].y+v[i+2].x*v[i].y-(v[i].y*v[i+1].x+v[i+1].y*v[i+2].x+v[i+2].y*v[i].x);
    aria/=2.0;
    if(aria<0)
    aria*=-1;
    fout<<fixed<<setprecision(8)<<aria;
    return 0;
}