#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1);
int main(){
  cout << fixed << setprecision(20);
  int N;
  long long L;
  cin >> N >> L;
  vector<long long> T(N);
  for (int i = 0; i < N; i++){
    cin >> T[i];
  }
  vector<double> t(N);
  for (int i = 0; i < N; i++){
    t[i] = (double) T[i] / L;
  }
  double sum = 0;
  long long cnt = 0;
  for (int i = 0; i < N; i++){
    for (int j = 0; j < N; j++){
      if (i != j){
        int m = N - 1 - (j + N - i) % N;
        sum += sin((t[j] - t[i]) * 2 * PI) / 2 * m;
        cnt += m;
      }
    }
  }
  sum /= cnt;
  sum *= 3;
  cout << sum << endl;
}