import math
import sys
def S(): return sys.stdin.readline().rstrip()
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int, sys.stdin.readline().rstrip().split())
def LI(): return list(map(int, sys.stdin.readline().rstrip().split()))
def LS(): return list(sys.stdin.readline().rstrip().split())

n = I()
x = LI()
m = I()

from statistics import pstdev, mean

st = pstdev(x)
me = mean(x)

ans = (x[m-1]-me)*10/st+50
print(ans)