import sys
readline=sys.stdin.readline
write=sys.stdout.write
from math import gcd as GCD
import math

N=int(readline())
X=list(map(int,readline().split()))
m=sum(X)//N
s=math.sqrt(sum((x-m)**2 for x in X)/N)
M=int(readline())
M-=1
if s==0:
    ans=50
else:
    ans=(X[M]-m)*10/s+50
print(ans)