import sys
input = sys.stdin.readline

N = int(input())
x = list(map(int, input().split()))
M = int(input())-1

if x==x[0]*N:
    print(50)
    exit()
    
ave = sum(x)/N
var = sum((xi-ave)**2 for xi in x)/N
sigma = var**0.5
d = (x[M]-ave)*10/sigma
print(int(50+d))