def main():
    n, m = map(int, input().split())
    n = float(n)
    b = list(map(float, input().split()))

    if m == 1:
        print(n - 1.0)
        return

    cumsum = [0.0] * m
    cumsum[m-1] = n - b[m-1]
    for i in range(1, m):
        cnt = m/i + cumsum[m-i]/i
        cumsum[m-i-1] = cumsum[m-i] + min(n-b[m-i-1],cnt)

    print(min(n-1.0, b[0]-1.0+cumsum[0]-cumsum[1]))

main()