n,m = map(int,input().split())
B = list(map(int,input().split()))
dp = [10**20]*m
dp[-1] = n-B[-1]
cum = dp[-1]
for i in range(m-1)[::-1]:
    dp[i] = B[i+1]-B[i]+dp[i+1]
    cal = m/(m-i-1)+cum/(m-i-1)
    if cal < dp[i]:
        dp[i] = cal
    cum += dp[i]
ans = dp[0]+B[0]-1
print(ans)