#include <fstream>
#include <iomanip>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

int n,i;
double aria;
struct puncte
{
    double x,y;
}v[100010];

int main()
{
    fin>>n;
    for(i=1;i<=n;++i)
        fin>>v[i].x>>v[i].y;
    v[n+1].x=v[1].x;
    v[n+1].y=v[1].y;
    for(i=1;i<=n;++i)
        aria+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    fout<<fixed<<setprecision(6)<<aria/2;
    return 0;
}