#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ld = long double;
#define fi first
#define se second
#define pb push_back

const double pi=3.141592653589793;

int main(){
  int n;
  int l;
  cin>>n>>l;

  int t[n];
  for(int i=0;i<n;i++){
    cin>>t[i];     
  } 
  
  double ans=0,p;
  
  for(int i=0;i<n;i++){
    for(int j=i+1;j<n;j++){
      p=2*(t[j]-t[i]);
      p/=l;
      p*=pi;
      ans+=(2*j-2*i-n)*sin(p);
    }
  }
  ans*=-3;
  ans/=n;
  ans/=n-1;
  ans/=n-2;
  
  cout<<fixed<<setprecision(15);
  cout<<ans<<endl;
}