#include #include using namespace std; #define rep(i,n) for (int i = 0; i < (n); ++i) #define Inf 1000000001 double theta[5001],r[5001]; int main(){ int N; cin>>N; long long L; cin>>L; double PI = acos(-1.0); vector x(N),y(N); rep(i,N){ long long T; cin>>T; x[i] = cos(PI * 2.0 / L * T); y[i] = sin(PI * 2.0 / L * T); } double ans = 0.0; rep(i,N){ double temp = 0.0; int pos = i+1; if(pos==N)pos = 0; double sp = 0.0,cp = 0.0,sm = 0.0,cm = 0.0; int cntp = 0,cntm = 0; int jj = i; rep(j,N-1){ jj++; if(jj>=N)jj = 0; double ss = y[jj]-y[i]; double cc = x[jj]-x[i]; double ttt = 0.0; if(j!=0){ ttt += ss * cp; ttt -= cc * sp; } temp += ttt; sp += ss; cp += cc; cntp ++; } temp /= N-1; temp *= 2.0; temp /= N-2; ans += temp/N; } ans /= 2.0; cout<