N = int(input())
X = list(map(int, input().split()))
M = int(input())

Ans = 0
A = sum(X) / N
X2 = [(i-A)**2 for i in X]
A2 = (sum(X2) / N)**0.5
if A2 == 0:
    Ans = 50
else:
    A3 = (abs(A-X[M-1])*10) / A2
    Ans = 50 + [-A3, A3][X[M-1] > A]

print(int(Ans))