//#define NDEBUG
#include <algorithm>
#include <cstddef>
#include <cstdint>
#include <iostream>
#include <vector>

namespace n91 {

using i8 = std::int_fast8_t;
using i32 = std::int_fast32_t;
using i64 = std::int_fast64_t;
using u8 = std::uint_fast8_t;
using u32 = std::uint_fast32_t;
using u64 = std::uint_fast64_t;
using isize = std::ptrdiff_t;
using usize = std::size_t;

struct rep {
  struct itr {
    usize i;
    constexpr itr(const usize i) noexcept : i(i) {}
    void operator++() noexcept { ++i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  const itr f, l;
  constexpr rep(const usize f, const usize l) noexcept
      : f(std::min(f, l)), l(l) {}
  constexpr auto begin() const noexcept { return f; }
  constexpr auto end() const noexcept { return l; }
};
struct revrep {
  struct itr {
    usize i;
    constexpr itr(const usize i) noexcept : i(i) {}
    void operator++() noexcept { --i; }
    constexpr usize operator*() const noexcept { return i; }
    constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
  };
  const itr f, l;
  constexpr revrep(const usize f, const usize l) noexcept
      : f(l - 1), l(std::min(f, l) - 1) {}
  constexpr auto begin() const noexcept { return f; }
  constexpr auto end() const noexcept { return l; }
};
template <class T> auto md_vec(const usize n, const T &value) {
  return std::vector<T>(n, value);
}
template <class... Args> auto md_vec(const usize n, Args... args) {
  return std::vector<decltype(md_vec(args...))>(n, md_vec(args...));
}
template <class T> constexpr T difference(const T &a, const T &b) noexcept {
  return a < b ? b - a : a - b;
}
template <class T> void chmin(T &a, const T &b) noexcept {
  if (b < a)
    a = b;
}
template <class T> void chmax(T &a, const T &b) noexcept {
  if (a < b)
    a = b;
}
template <class T> T scan() {
  T ret;
  std::cin >> ret;
  return ret;
}

} // namespace n91
#include <cstdint>

namespace n91 {

constexpr std::uint_fast64_t totient(std::uint_fast64_t x) noexcept {
  using u64 = std::uint_fast64_t;
  u64 ret = x;
  for (u64 i = static_cast<u64>(2); i * i <= x; ++i) {
    if (x % i == static_cast<u64>(0)) {
      ret -= ret / i;
      x /= i;
      while (x % i == static_cast<u64>(0)) {
        x /= i;
      }
    }
  }
  if (x != static_cast<u64>(1)) {
    ret -= ret / x;
  }
  return ret;
}

template <std::uint_fast64_t Modulus,
          std::uint_fast64_t InverseExp =
              totient(Modulus) - static_cast<std::uint_fast64_t>(1)>
class modint {
  using u64 = std::uint_fast64_t;

  static_assert(Modulus < static_cast<u64>(1) << static_cast<u64>(32),
                "Modulus must be less than 2**32");

  u64 a;

  constexpr modint &negate() noexcept {
    if (a != static_cast<u64>(0)) {
      a = Modulus - a;
    }
    return *this;
  }

public:
  constexpr modint(const u64 x = static_cast<u64>(0)) noexcept
      : a(x % Modulus) {}
  constexpr u64 &value() noexcept { return a; }
  constexpr const u64 &value() const noexcept { return a; }
  constexpr modint operator+() const noexcept { return modint(*this); }
  constexpr modint operator-() const noexcept { return modint(*this).negate(); }
  constexpr modint operator+(const modint rhs) const noexcept {
    return modint(*this) += rhs;
  }
  constexpr modint operator-(const modint rhs) const noexcept {
    return modint(*this) -= rhs;
  }
  constexpr modint operator*(const modint rhs) const noexcept {
    return modint(*this) *= rhs;
  }
  constexpr modint operator/(const modint rhs) const noexcept {
    return modint(*this) /= rhs;
  }
  constexpr modint &operator+=(const modint rhs) noexcept {
    a += rhs.a;
    if (a >= Modulus) {
      a -= Modulus;
    }
    return *this;
  }
  constexpr modint &operator-=(const modint rhs) noexcept {
    if (a < rhs.a) {
      a += Modulus;
    }
    a -= rhs.a;
    return *this;
  }
  constexpr modint &operator*=(const modint rhs) noexcept {
    a = a * rhs.a % Modulus;
    return *this;
  }
  constexpr modint &operator/=(modint rhs) noexcept {
    u64 exp = InverseExp;
    while (exp) {
      if (exp % static_cast<u64>(2) != static_cast<u64>(0)) {
        *this *= rhs;
      }
      rhs *= rhs;
      exp /= static_cast<u64>(2);
    }
    return *this;
  }
  constexpr bool operator==(const modint rhs) const noexcept {
    return a == rhs.a;
  }
  constexpr bool operator!=(const modint rhs) const noexcept {
    return a != rhs.a;
  }
};

template <class T, std::uint_fast64_t v> class modint_constant {
public:
  static constexpr T value = static_cast<T>(v);

  using value_type = T;
};
template <class T, std::uint_fast64_t v>
constexpr T modint_constant<T, v>::value;

} // namespace n91
#include <vector>

namespace n91 {

template <class T> class fact_binom {
public:
  using value_type = T;
  using container_type = std::vector<value_type>;
  using size_type = typename container_type::size_type;

private:
  container_type factrial, inv_fact;

public:
  fact_binom() : factrial(), inv_fact() {}
  explicit fact_binom(const size_type n) : factrial(n + 1), inv_fact(n + 1) {
    factrial[0] = static_cast<value_type>(1);
    for (size_type i = 0; i != n; ++i) {
      factrial[i + 1] = static_cast<value_type>(i + 1) * factrial[i];
    }
    inv_fact[n] = static_cast<value_type>(1) / factrial[n];
    for (size_type i = n; i != 0; --i) {
      inv_fact[i - 1] = inv_fact[i] * static_cast<value_type>(i);
    }
  }

  value_type operator()(const size_type n, const size_type r) const {
    return factrial[n] * inv_fact[r] * inv_fact[n - r];
  }
};

} // namespace n91

#include <algorithm>
#include <complex>
#include <iomanip>
#include <iostream>
#include <limits>
#include <utility>

namespace n91 {

u64 rsub(const u64 l, const u64 r) { return l < r ? r - l : 0; }

void main_() {
  /*
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  //*/
  const usize n = scan<usize>();
  using v = std::complex<double>;
  std::vector<v> p(n);
  for (auto &e : p) {
    e.real(scan<double>());
    e.imag(scan<double>());
  }
  double ans = 0;
  for (const usize i : rep(0, n)) {
    for (const usize j : rep(0, i)) {
      ans += std::abs(p[i] - p[j]);
    }
  }
  std::cout << std::setprecision(20) << ans * 2 / n << std::endl;
}

} // namespace n91

int main() {
  n91::main_();
  return 0;
}


Measured memory_bytes: 256