import math
import sys

def find_m_y(m_x, n_list):
	m_y = m_x
	for m in n_list:
		m_y += m
	return m_y

K = int(input())

m_x = float(input())

measurements = 0
for i in range(K):

	n_list = list(map(float, input().split()))
	n_list_mod = []
	n = n_list.pop(0)

	m_y = find_m_y(m_x, n_list)
	measurements += m_y

print(measurements / K)