def aa(s):
    global m
    c=s+1
    res=m/(m-c)+b[c]/(m-c)+a[0]-1
    return res

n,m=map(int,input().split())
a=list(map(int,input().split()))
b=[0]*(m+1)
for i in range(m-1,-1,-1):
    b[i]=b[i+1]+n-a[i]
q,w=0,m-1
while w-q>2:
    e,r=q+(w-q)//3,q+(w-q)//3*2
    if aa(e)<aa(r):
        w=r
    else:
        q=e
ans=float(n-1)
for i in range(q,w):
    ans=min(ans,aa(i))
print(ans)