#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#include <stack>
#include <array>
#include <list>
#define popcount __builtin_popcount
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;
const double PI=acos(-1.0);
ll t[500050];
int main()
{
    int n; ll l; cin>>n>>l;
    for(int i=0; i<n; i++){
        scanf("%lld", &t[i]);
    }
    double ans=0;
    double sc=0, ss=0, sc1=0, ss1=0;
    for(int i=0; i<n; i++){
        double z=2*PI*t[i]/l;
        ans+=n*(sin(z)*sc-cos(z)*ss);
        ans-=2*i*sin(z)*sc;
        ans+=2*i*cos(z)*ss;
        ans+=2*sin(z)*sc1;
        ans-=2*cos(z)*ss1;
        sc+=cos(z);
        ss+=sin(z);
        sc1+=i*cos(z);
        ss1+=i*sin(z);
    }
    /*
    for(int i=0; i<n; i++){
        for(int j=0; j<i; j++){
            ans+=(n-2*i+2*j)*sin(PI*2*(t[i]-t[j])/l);
        }
    }
    */
    ans/=((double)n*(n-1)*(n-2)/3);
    printf("%.11lf\n", ans);
    return 0;
}