n,m = map(int,input().split())
*b, = list(map(int,input().split()))
dp = [0.0]*m
acc = [0.0]*m
dp[-1] = acc[-1] = n-b[-1]
for i in range(m-1)[::-1]:
    dp[i] = min(n-b[i], m/(m-1-i) + acc[i+1]/(m-1-i))
    acc[i] = acc[i+1] + dp[i]
print(dp[0]+b[0]-1)