#include <algorithm>
#include <cmath>
#include <cstdio>

using namespace std;

#define rep(i, n) for (size_t i = 0; i < (n); i++)

template<typename T>
T sq(T x) {
  return x * x;
}

int main() {
  size_t N;
  scanf("%ld", &N);
  pair<double, double> cs[N];
  rep(i, N) scanf("%lf%lf", &cs[i].first, &cs[i].second);
  // (\sum_{i<j} d_{i,j} * (n-1) * n! / \binom{n}{2}) / n!
  // = (\sum_{i<j} d_{i,j}) * (n-1) / \binom{n}{2}
  // = (\sum_{i<j} d_{i,j}) * (n-1) / (n * (n-1) / 2)
  // = (\sum_{i<j} d_{i,j}) * 2 / n
  double d = 0.0;
  rep(i, N - 1)
    for (size_t j = i; j < N; j++)
      d += sqrt(sq(cs[j].first - cs[i].first) + sq(cs[j].second - cs[i].second));
  printf("%.7lf\n", d * 2 / N);
  return 0;
}
