import math
N = int(input())
X = list(map(int, input().split()))
M = int(input())
a = sum(X)/N
v = []
for x in X:
    v.append((x-a)**2)
u = math.sqrt(sum(v)/N)
if u==0:
    print(50)
    exit()
t = abs(X[M-1]-a)*10/u
if X[M-1]>=a:
    print(int(50+t))
else:
    print(int(50-t))