N, M = map(int, input().split())

B = list(map(int, input().split()))

minB = min(B)
maxB = max(B)

cnt = minB - 1

W =[]
for b in B:
    if maxB - b < M and b - minB > M:
        W.append(N-b)
        
if len(W) > 0:
    cnt += (sum(W)+M)/len(W)
else:
    cnt += N - minB
        
print(float(cnt))