// https://atcoder.jp/contests/past-sample/tasks/abc129_d

// config 

const int __SETW__ = 6;

// IO
#include <iostream>      // cin, cout
#include <iomanip>       // setw
// MATH & ALGROTHM
#include <cmath>
#include <complex>
#include <numeric>
#include <algorithm>
// CONTAINER 
#include <tuple>
#include <string>      
#include <vector>
#include <deque>
#include <queue>  
#include <stack>
#include <map>       
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <list>
// FUNCTION
#include <functional>
// META
#include <type_traits>
 
/////// 
using std::cin;
using std::cout;
using std::endl;

using INT = long long int;
const INT INF = INT(1) << 60;

#define FOR(i, N) for( INT i = 0; i < N; i++ )
#define RANGE(v) begin(v), end(v)

using VEC = std::vector<INT>;
using VECVEC = std::vector<VEC>;

template< class T >
T safe_get ( const std::vector<T>& v, INT idx, T def = -1 ){
    if( idx < 0 || v.size() <= idx ){
        return def;
    } else {
        return v[idx];
    }
}
template< class T >
T safe_get ( const std::vector< std::vector<T> >& vv, INT idx1, INT idx2, T def = -1 ){
    if( idx1 < 0 || vv.size() <= idx1 ){
        return def;
    } else {
        return safe_get(vv[idx1], idx2, def );
    }
}

template< class T, class F >
inline bool ANY( T sequence, F func ){
    for(  auto&& t : sequence ){
        if( func(t) ){ return true; }
    }
    return false;
}
template< class T, class F >
inline bool ALL( T sequence, F func ){
    for(  auto&& t : sequence ){
        if( ! func(t) ){ return false; }
    }
    return true;
}

template< class T >
std::ostream& operator<< ( std::ostream& lhs, const std::priority_queue<T>& rhs ){
    std::queue<T> copy = rhs;
    while( !copy.empty() ){
        lhs << std::setw(__SETW__) << copy.front() << " ";
        copy.pop();
    }
    return lhs;
}
template< class T >
std::ostream& operator<< ( std::ostream& lhs, const std::queue<T>& rhs ){
    std::queue<T> copy = rhs;
    while( !copy.empty() ){
        lhs << std::setw(__SETW__) << copy.front() << " ";
        copy.pop();
    }
    return lhs;
}
template< class T >
std::ostream& operator<< ( std::ostream& lhs, const std::stack<T>& rhs ){
    std::stack<T> copy = rhs;
    while( !copy.empty() ){
        lhs << std::setw(__SETW__) << copy.top() << " ";
        copy.pop();
    }
    return lhs;
}

template< class T >
std::ostream& operator<< ( std::ostream& lhs, const std::vector< std::vector<T> >& rhs ){
    for( auto&& v : rhs ){
        lhs << v << endl;
    }
    return lhs;
}
template< class T >
std::ostream& operator<< ( std::ostream& lhs, const std::vector<T>& rhs ){
    for( auto&& t : rhs ){
        lhs << std::setw(__SETW__) << t << " ";
    }
    return lhs;
}
template< class T >
std::ostream& operator<< ( std::ostream& lhs, const std::list<T>& rhs ){
    for( auto&& t : rhs ){
        lhs << std::setw(__SETW__) << t << " ";
    }
    return lhs;
}

template< class T1, class T2 >
std::ostream& operator<< ( std::ostream& lhs, const std::map<T1, T2>& rhs ){
    for( auto&& m : rhs ){
        lhs << std::setw(__SETW__) << m.first << ":" << std::setw(__SETW__) << m.second << endl;
    }
    return lhs;
}
template< class T1, class T2 >
std::ostream& operator<< ( std::ostream& lhs, const std::multimap<T1, T2>& rhs ){
    for( auto&& m : rhs ){
        lhs << std::setw(__SETW__) << m.first << ":" << std::setw(__SETW__) << m.second << endl;
    }
    return lhs;
}
template< class T >
std::ostream& operator<< ( std::ostream& lhs, const std::set<T>& rhs ){
    for( auto&& t : rhs ){
        lhs << std::setw(__SETW__) << t << " ";
    }
    return lhs;
}
template< class T >
std::ostream& operator<< ( std::ostream& lhs, const std::multiset<T>& rhs ){
    for( auto&& t : rhs ){
        lhs << std::setw(__SETW__) << t << " ";
    }
    return lhs;
}

template< class T >
std::ostream& operator<< ( std::ostream& lhs, const std::complex<T>& rhs ){
    std::streamsize ss = lhs.precision();
    lhs << "(" << std::setw(5) << std::setprecision(4) << rhs.real() << ","
               << std::setw(5) << std::setprecision(4) << rhs.imag() << ") ";
    lhs.precision(ss);
    return lhs;
}
template< class T1, class T2 >
std::ostream& operator<< ( std::ostream& lhs, const std::pair<T1, T2>& rhs ){
    lhs << "(" << rhs.first << "," << rhs.second << ")";
    return lhs;
}

template< class T >
T chmax( T& lhs, T& rhs){
    lhs = std::max( lhs, rhs);
    return lhs;
}
template< class T >
T chmin( T& lhs, T& rhs){
    lhs = std::min( lhs, rhs);
    return lhs;
}


int main(){
    INT N;
    cin >> N;

    std::vector<double> x(N);
    std::vector<double> y(N);

    FOR(i, N) cin >> x[i] >> y[i];

    std::vector<int> idxs(N); 
    FOR(i, N) idxs[i] = i; 

    double ans = 0;
    INT count = 0;
    do {
        double ans2 = 0;
        // cout << idxs << endl;
        FOR(i, N-1){
            double dx = x[idxs[i]] - x[idxs[i+1]];
            double dy = y[idxs[i]] - y[idxs[i+1]];
            ans2 += sqrt( dx*dx + dy*dy );
        }
        count++;
        ans += ans2;
    } while( std::next_permutation( RANGE(idxs)));

    cout << std::setprecision(16) << ans / double(count) << endl;

    return 0;
}


Measured memory_bytes: 256