import sys, math
 
sys.setrecursionlimit(10 ** 6)
stdin = sys.stdin

ni = lambda: int(ns())
na = lambda: list(map(int, stdin.readline().split()))
ns = lambda: stdin.readline().strip()
nsa = lambda: list(stdin.readline().split())
mod = 10 * 7 + 7

n = ni()
X = na()
m = ni()

ave = sum(X) / n
s2 = 0
for x in X:
    s2 += (abs(ave - x)) ** 2

s2 /= n
s = math.sqrt(s2)
if s != 0:
    ans = 50 + (X[m - 1] - ave) * 10 / s 
else:
    ans = 50

print(int(ans))