n,m=map(int,input().split())
b=[-1]+list(map(int,input().split()))
def f(s):
    dp=[10**10]*(m+1)
    dp[m]=n-b[m]
    for i in range(m-1,0,-1):
        dp[i]=min(dp[i+1]+b[i+1]-b[i],(s)/m+1)
    return sum(dp[i] for i in range(1,m+1))/m

mi=0
ma=m*n+1
while ma/mi-1>10**(-7):
    mid=(ma+mi)/2
    if f(mid)-mid<0:ma=mid
    else:mi=mid

s=ma
dp=[10**10]*(m+1)
dp[m]=n-b[m]
for i in range(m-1,0,-1):
    dp[i]=min(dp[i+1]+b[i+1]-b[i],s/m+1)
print(dp[1]+b[1]-1)