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

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

cnt = B[0] - 1 + N - B[-1]

avg = sum(B)/M
W =[]
minExp = B[-1]-B[0]
for k in range(1, M):
    exp = M / (M-k) + 1 / (M-k) * (B[-1]*(M-k) - sum(B[k:]))
    minExp = min(minExp, exp)
        
print(float(cnt+minExp))