#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")

#include <iostream>
#include <cmath>
using namespace std;
typedef long double ld;
ld t[500010],pi = acos(-1),si[500010],co[500010];
int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.precision(20);
    int i,n; ld l;
    cin >> n >> l;
    for(i=0;i<n;i++){
        cin >> t[i];
        si[i] = sin(2*pi*t[i]/l); co[i] = cos(2*pi*t[i]/l);
    }
    ld s = 0,c = 0,ans = 0;
    ld ans2 = 0;
    for(i=n - 1;i>=0;i--){
        ans += ld(n + 2*i)*(s*co[i] - c*si[i]);
        s += si[i]; c += co[i];
    }
    s = 0; c = 0;
    for(i=0;i<n;i++){
        ans += ld(2*i)*(s*co[i] - c*si[i]);
        s += si[i]; c += co[i];
    }
    ans *= 3;
    ans /= (ld)(n*(n - 1)*(n - 2));
    cout << ans << endl;
}