# import sys; input = sys.stdin.buffer.readline
# sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

#処理内容
def main():
	N = int(input())
	X = getlist()
	M = int(input())
	ave = sum(X) / N
	s2 = 0
	for i in range(N):
		s2 += (X[i] - ave) ** 2

	if s2 == 0:
		print(50)
		return
	s = (s2 / N) ** 0.5
	# print(ave, s2, s)


	ans = 50 + (X[M - 1] - ave) * 10 / s
	if ans >= 0:
		print(int(ans // 1))
	else:
		ans2 = -int(abs(ans) // 1)
		print(ans2)


	# if X[M - 1] >= ave:
	# 	dif = int()
	# 	print(50 + dif)
	# else:
	# 	dif = int((ave - X[M - 1]) * 10 // s)
	# 	print(50 - dif)





if __name__ == '__main__':
	main()