N,M=map(int, input().split())
A=list(map(int, input().split()))
if M==1:
  print(N-1)
  exit()
b=A[0]-1+(N-A[-1])
cnt=M
ans=N-1
def f(a,r):
  d=a/(1-r)
  return d 
e=0
cnt=1
for i in range(M-1,0,-1):
  r=i/M
  g=f(1,r)
  e+=A[-1]-A[i]
  ff=g+e/cnt
  cnt+=1
  ans=min(ans,b+ff)
print(ans)