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

cum = [0]

for i in range(M):
    cum.append(cum[-1] + B[i])

res = N - 1

for i in range(M - 1):
    cnt = B[0] - 1
    cnt += 1 / (1 - (i + 1) / M)
    cnt += N - (cum[M] - cum[i + 1]) / (M - i - 1)
    res = min(res, cnt)

print(res)