n, m = map(int, input().split())
b = list(map(int, input().split()))
l = [0]
for i in range(m):
    l.append(l[-1] + b[i])
ans = n - 1
for i in range(m - 1):
    ans = min(ans, b[0] - 1 + 1 / (1 - (i + 1) / m) + n - (l[m] - l[i + 1]) / (m - i - 1))
print(ans)