Language: C++

\#include <bits/stdc++.h>
typedef long long ll;
using namespace std;

int kaijo(int a){
    if( a != 0){
        return a*(kaijo(a-1));
    }

    return 1;
}

int main(){
    int n; cin>>n;
    vector<pair<int,int>> p(n);
    for(int i=0; i<n; i++)cin>>p[i].first>>p[i].second;
    double sum=0;
    sort(p.begin(),p.end());
    do{
        for(int i=0; i<n-1; i++){
            sum += sqrt((p[i].first-p[i+1].first)*(p[i].first-p[i+1].first)+
            (p[i].second-p[i+1].second)*(p[i].second-p[i+1].second));
        }

    }while(next_permutation(p.begin(),p.end()));
    
    cout<<setprecision(6)<<fixed;
    cout<<(double)sum/kaijo(n)<<endl;

    return 0;
}

Compiler output:
a.cc:1:1: error: stray '\' in program
    1 | \#include <bits/stdc++.h>
      | ^
a.cc:1:2: error: stray '#' in program
    1 | \#include <bits/stdc++.h>
      |  ^
a.cc:1:3: error: 'include' does not name a type
    1 | \#include <bits/stdc++.h>
      |   ^~~~~~~
a.cc: In function 'int main()':
a.cc:14:12: error: 'cin' was not declared in this scope
   14 |     int n; cin>>n;
      |            ^~~
a.cc:1:1: note: 'std::cin' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
  +++ |+#include <iostream>
    1 | \#include <bits/stdc++.h>
a.cc:15:5: error: 'vector' was not declared in this scope
   15 |     vector<pair<int,int>> p(n);
      |     ^~~~~~
a.cc:1:1: note: 'std::vector' is defined in header '<vector>'; this is probably fixable by adding '#include <vector>'
  +++ |+#include <vector>
    1 | \#include <bits/stdc++.h>
a.cc:15:12: error: 'pair' was not declared in this scope
   15 |     vector<pair<int,int>> p(n);
      |            ^~~~
a.cc:1:1: note: 'std::pair' is defined in header '<utility>'; this is probably fixable by adding '#include <utility>'
  +++ |+#include <utility>
    1 | \#include <bits/stdc++.h>
a.cc:15:17: error: expected primary-expression before 'int'
   15 |     vector<pair<int,int>> p(n);
      |                 ^~~
a.cc:16:32: error: 'p' was not declared in this scope
   16 |     for(int i=0; i<n; i++)cin>>p[i].first>>p[i].second;
      |                                ^
a.cc:18:10: error: 'p' was not declared in this scope
   18 |     sort(p.begin(),p.end());
      |          ^
a.cc:18:5: error: 'sort' was not declared in this scope; did you mean 'short'?
   18 |     sort(p.begin(),p.end());
      |     ^~~~
      |     short
a.cc:21:20: error: 'sqrt' was not declared in this scope
   21 |             sum += sqrt((p[i].first-p[i+1].first)*(p[i].first-p[i+1].first)+
      |                    ^~~~
a.cc:25:12: error: 'next_permutation' was not declared in this scope
   25 |     }while(next_permutation(p.begin(),p.end()));
      |            ^~~~~~~~~~~~~~~~
a.cc:27:5: error: 'cout' was not declared in this scope
   27 |     cout<<setprecision(6)<<fixed;
      |     ^~~~
a.cc:27:5: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:27:11: error: 'setprecision' was not declared in this scope
   27 |     cout<<setprecision(6)<<fixed;
      |           ^~~~~~~~~~~~
a.cc:1:1: note: 'std::setprecision' is defined in header '<iomanip>'; this is probably fixable by adding '#include <iomanip>'
  +++ |+#include <iomanip>
    1 | \#include <bits/stdc++.h>
a.cc:27:28: error: 'fixed' was not declared in this scope
   27 |     cout<<setprecision(6)<<fixed;
      |                            ^~~~~
a.cc:1:1: note: 'std::fixed' is defined in header '<ios>'; this is probably fixable by adding '#include <ios>'
  +++ |+#include <ios>
    1 | \#include <bits/stdc++.h>
a.cc:28:33: error: 'endl' was not declared in this scope
   28 |     cout<<(double)sum/kaijo(n)<<endl;
      |                                 ^~~~
a.cc:1:1: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
  +++ |+#include <ostream>
    1 | \#include <bits/stdc++.h>