#include <bits/stdc++.h>
//#include <chrono>
//#pragma GCC optimize("Ofast")
using namespace std;
#define reps(i,s,n) for(int i = s; i < n; i++)
#define rep(i,n) reps(i,0,n)
#define Rreps(i,n,e) for(int i = n - 1; i >= e; --i)
#define Rrep(i,n) Rreps(i,n,0)
#define ALL(a) a.begin(), a.end()

using ll = long long;
using vec = vector<ll>;
using mat = vector<vec>;

ll N,M,H,W,Q,K,A,B;
string S;
using P = pair<ll, ll>;
const ll INF = (1LL<<60);

template<class T> bool chmin(T &a, const T b){
    if(a > b) {a = b; return true;}
    else return false;
}
template<class T> bool chmax(T &a, const T b){
    if(a < b) {a = b; return true;}
    else return false;
}
template<class T> void my_printv(std::vector<T> v,bool endline = true){
    if(!v.empty()){
        for(std::size_t i{}; i<v.size()-1; ++i) std::cout<<v[i]<<" ";
        std::cout<<v.back();
    }
    if(endline) std::cout<<std::endl;
}

using ld = long double;
using vd = vector<long double>;

const ld pi = acos(-1);
int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cin>>N>>K;
    vd t(N);
    vd s(N), c(N);
    rep(i, N){
        cin>>t[i];
        s[i] = sin(t[i] * 2.0 * pi/ K);
        c[i] = cos(t[i] * 2.0 * pi/ K);
    }
    vd ss(N + 1), sc(N + 1);
    ld res = 0, sum_ij = 0, ns = 0, nc = 0;
    cout<<fixed<<setprecision(15);
    rep(i, N){
        res += sum_ij + s[i] * (nc * 2 - sc[i] * (i - 1)) + c[i] * (ss[i] * (i - 1) - ns * 2);
        ss[i+1] = ss[i] + s[i];
        sc[i+1] = sc[i] + c[i];
        ns += s[i] * i;
        nc += c[i] * i;
        sum_ij += sc[i+1] * s[i] - ss[i+1] * c[i];
    }
    cout<<res * 3 / (N * (N - 1) * (N - 2))<<endl;
}