n, m = map(int, input().split())
b = list(map(int, input().split()))

if m == 1:
    print(n - 1)
else:
    ans = float(b[-1] - b[0])
    sum = 0.0
    for i in range(m - 1):
        sum += b[-1] - b[m - 1 - i]
        ans = min(ans, (sum + m) / float(i + 1))
    ans += b[0] + n - 1 - b[-1]
    print("{:.8f}".format(ans))