n = int(input())
a = list(map(int, input().split()))

a_sum = 0
for i in a:
    a_sum += i

a_ave = a_sum / n

a_sd_sum = 0
for i in a:
    a_sd_sum += (i - a_ave)**2

a_sd = (a_sd_sum / n)**0.5

if a_sd == 0:
    print(50)
else:
    print(int(50 + (a[(int(input())) - 1] - a_ave) * 10 / a_sd))