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