#include<fstream>
#include<math.h>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct punct {
    double x, y;
} v[100005];
int n,i;
double sol;
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
        f>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for(i=1;i<=n;i++)
        sol+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    sol/=(double)2;
    if(sol<0)
        g<<-sol;
    else
        g<<sol;
}