N, P = map(int, input().split())
INF = 10**10
dp = [[INF] * (P+1) for i in range(2)]
dp[0][0] = 0
for i in range(N):
    dp[(i+1)%2][:] = dp[i%2][:]
    scores = list(map(int, input().split())) + [1]    
    # 今まで解いた問題の合計 j
    for j in range(P+1):
        # このコンテストで解いた問題数 k
        tmp = INF
        for k,s in enumerate(scores):
            if j - k >= 0:
                tmp = min(dp[i%2][j-k]+s,tmp)
        dp[(i+1)%2][j] = tmp
print(dp[N%2][P]/N)