#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; }