N, M = map(int, input().split())
B = list(map(int, input().split()))
val = B[0] - 1 + N - B[-1]
S = 0
ans = N - 1
for k in range(M - 1, 0, -1):
    S += B[-1] - B[k]
    temp = (M + S)/(M - k)
    ans = min(ans, temp + val)
print(ans)