n = int(input())
students = list(map(int, input().split()))
m = int(input())

ave = sum(students) / n

var = sum((student - ave) ** 2 for student in students)

std = (var / n) ** 0.5

if std == 0:
    print(50)
    exti()
else:
    print(int(((students[m - 1] - ave) / std) * 10 + 50))