#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;

ll inputinf=1e12;

int main(){
  
  int n;
  ll l;
  cin>>n>>l;
  
  if(n>500000){
    cout<<"WARNING : N"<<endl;
  }
  if(l>inputinf){
    cout<<"WARNING : L"<<endl;
  }

  ll t[n];
  double tsin[n],tcos[n],tsinsum[n+1],tcossum[n+1],p;
  tsinsum[0]=0,tcossum[0]=0;
  
  for(int i=0;i<n;i++){
    cin>>t[i];
    
    if(t[i]>=l||t[i]<0){
      cout<<"WARNING : t["<<i<<"]"<<endl;
    }
    if(i>1){
      if(t[i-1]>=t[i]){
        cout<<"WARNING : t["<<i<<"]"<<endl;
      }
    }
    
    p=2*t[i];
    p*=pi;
    p/=l;
    tsin[i]=sin(p),tcos[i]=cos(p);
    tsinsum[i+1]=tsinsum[i]+tsin[i];
    tcossum[i+1]=tcossum[i]+tcos[i];
  }
  
  double sinsum=0,cossum=0;
  for(int i=1;i<n;i++){
    sinsum+=tsin[i]*(i-1);
    cossum+=tcos[i]*(i-1);
  }
  
  double ans=0;
  
  for(int i=0;i<n;i++){
    ans+=tsin[i]*cossum-tcos[i]*sinsum;
    sinsum-=tsinsum[n],cossum-=tcossum[n];
    sinsum+=tsin[i]*(n-1)+tsin[(i+1)%n],cossum+=tcos[i]*(n-1)+tcos[(i+1)%n];
  }
  
  ans*=3;
  ans/=n;
  ans/=n-1;
  ans/=n-2;
  cout<<fixed<<setprecision(15);
  cout<<ans<<endl;
}