import math
N = int(input())
X = list(map(int, input().split()))
M = int(input())

ave = sum(X)/N

memo = 0
for x in X:
    memo += (x - ave)**2

if memo == 0:
    exit(print(50))

#memo = (memo/N)**(1/2)
memo = math.sqrt(memo/N)

x = X[M - 1]
ans = 50 + (10*(x - ave))/memo
ans = int(ans)
print(ans)