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