N,M = map(int, input().split())
B = list(map(int, input().split()))

dp = [float('inf')]*M
dp[-1] = N-B[-1]
sm = dp[-1]
for i in range(M-2,-1,-1):
    dp[i] = min(B[i+1]-B[i]+dp[i+1],(M+sm)/(M-i-1))
    sm += dp[i]
print(dp[0]+B[0]-1)