N = int(input())
X = list(map(int, input().split()))
M = int(input())
mu = sum(X) / N
var = sum([(X[i] - mu) ** 2 for i in range(N)]) / N
sigma = var ** 0.5
if sigma == 0:
    print(50)
else:
    print(50 + int(10 * (X[M - 1] - mu) / sigma))