#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n,P0;
long double S,x,y,x0,y0,xp,yp;
int main()
{
    f>>n;S=0;
    f>>xp>>yp;x0=xp;y0=yp;
    for(int i=1;i<n;++i)
    {
        f>>x>>y;
        S+=xp*y-x*yp;
        xp=x;yp=y;
    }
    S+=xp*y0-x0*yp;
    g<<fixed<<setprecision(10)<<(S>0?S:-S)/2;
}