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

ans=[n-1]
ct=0
for i in range(m-1,-1,-1):
  ct+=b[-1]-b[i]
  x=m/(m-i)+ct/(m-i)
  ans.append(x+(n-b[-1])+(b[0]-1))

print(min(ans))