n = int(input())
xlst = list(map(int, input().split()))
m = int(input())
point = xlst[m - 1]
ave = sum(xlst) / n
v = 0
for x in xlst:
    v += (x - ave) ** 2
v = (v / n) ** 0.5
if v == 0:
    print(50)
else:
    ans = 50 + (point - ave) * 10 / v
    print(int(ans))