#include <fstream>
#include<deque>
#include<algorithm>
#include<string>
#include<bits/stdc++.h>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

int main()
{
  int n;
  long double x1,y1,xi,yi,xj,yj, arie=0;
  fin>>n;
  fin>>x1>>y1;
  xi=x1;
  yi=y1;
  for(int i=1;i<n;i++){
     fin>>xj>>yj;
     arie+=(xi*yj-xj*yi);
     xi=xj;
     yi=yj;
  }
  arie+=(xi*y1-x1*yi);
  fout<<setprecision(6)<<fixed<<arie/2.0;
}