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

ans = N - 1
sum = 0
for k in range(1, M):
    E = (M + sum) / k
    E += N - B[-1]
    E += B[0] - 1
    ans = min(ans, E)
    sum += B[M-1] - B[M-k-1]

print("{:.12f}".format(ans))