#include "bits/stdc++.h"
using namespace std;
#define ALL(x) begin(x), end(x)
#define rep(i, n) for (int i = 0; i < (n); i++)
#define mod 1000000007
using ll = long long;
const int INF = 1000000000;
const ll LINF = 1001002003004005006ll;
int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template <class T>
bool chmax(T &a, const T &b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
template <class T>
bool chmin(T &a, const T &b) {
    if (b < a) {
        a = b;
        return true;
    }
    return false;
}

struct IOSetup {
    IOSetup() {
        cin.tie(0);
        ios::sync_with_stdio(0);
        cout << fixed << setprecision(12);
    }
} iosetup;

template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v) {
    for (int i = 0; i < (int)v.size(); i++) os << v[i] << (i + 1 == (int)v.size() ? "" : " ");
    return os;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &v) {
    for (T &x : v) is >> x;
    return is;
}
double pi = acos(-1);
signed main() {
    ll N, L; cin >> N >> L;
    vector<ll> T(N); cin >> T;
    for(int i = 0; i < N; i++) T.push_back(T[i] + L);

    double res = 0;
    for(int i = 0; i < N; i++){
        for(int k = 1; k < N; k++){
            int j = i + k;
            double theta = 2.0 * pi * (T[j] - T[i]) / L;
            double cnt   = N - (j - i) - 1;
            // cout << i << " - " << k << " : " << cnt << " * " << 0.5 * sin(theta) << endl;
            res += 0.5 * cnt * sin(theta);
        }
    }
    double den = double(N) * (N - 1) * (N - 2) / 6.0;
    cout << res / den << endl;
    return 0;
}