#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)
typedef long long ll;
#define int long long

double men(double x1,double y1,double x2,double y2,double x3,double y3){
  double menseki=0;
  double a1=sqrt(pow(abs(x2-x1),2)+pow(abs(y2-y1),2));
  double b1=sqrt(pow(abs(x3-x2),2)+pow(abs(y3-y2),2));
  double c1=sqrt(pow(abs(x3-x1),2)+pow(abs(y3-y1),2));
  double s=(a1+b1+c1)/2;
  menseki=sqrt(s*(s-a1)*(s-b1)*(s-c1));
  return menseki;
}

signed main(){
  int n;
  cin>>n;
  double x[n],y[n];
  rep(i,n){
    cin>>x[i]>>y[i];
  }
  double ans=0;
  rep(i,n-2){
    ans+=men(x[0],y[0],x[i+1],y[i+1],x[i+2],y[i+2]);
  }
  cout<<setprecision(10)<<ans*2<<endl;
  return 0;
}