from math import sqrt

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

m -= 1
mean = sum(A) / n
dif = 0
for i in range(n):
    dif += (A[i] - mean)**2
dif = sqrt(dif / n)
if dif == 0:
    print(50)
    exit()
print(int(50 + (A[m] - mean) * 10 / dif))