#include <fstream>

#define nMax 100005
#define x first
#define y second

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
pair<double, double> v[nMax];
double sol;
int n;
int main()
{
    fin>>n;
    for(int i=0;i<n;i++)
        fin>>v[i].x>>v[i].y;
    v[n]=v[0];
    for(int i=0;i<n;i++)
        sol+=(v[i].x*v[i+1].y - v[i+1].x*v[i].y);
    fout<<sol/2.0;
    return 0;
}