#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int modul( float s)
{
   return(max(s,-s));
}
int main()
{
    long long x,y,xf = 0,yf = 0,n,i,xi,yi;
    long long s = 0;
    f >> n ;
    f >> xi >> yi;
    xf = xi;
    yf = yi;
    for( i = 2; i <= n; i++)
    {
        f >> x >> y;
        s += xf*y - x*yf;
        xf = x;
        yf = y;
    }
    g <<  std::setprecision(5) << (s +  x*yi - xi*y) / 2;
    return 0;
}