import os,io,heapq
input=io.BytesIO(os.read(0,os.fstat(0).st_size)).readline
n,m=map(int,input().split())
a=list(map(int,input().split()))
b=[]
for i in range(m):
  b.append(n-a[i])
currsum=0
flag=0
for i in range(m-1,-1,-1):
  if (currsum+i+1)<=(m-i-1)*b[i]:
    print(a[0]-1+(currsum+m)/(m-i-1))
    exit()
  else:
    currsum+=b[i]
print(n-1)