#include <fstream>

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n,i;
double arie;
struct punctul
{
    double x,y;
} pct[100001] ;
int main()
{
    f>>n;
    for(i=0;i<n;i++)
        f>>pct[i].x>>pct[i].y;
    pct[n]=pct[0];
    for(i=0;i<n;i++)
        arie=arie+(pct[i].x*pct[i+1].y-pct[i+1].x*pct[i].y);
    if(arie<0) arie*=-1;
    g<<arie/2.0;
    return 0;
}