N, *X, M = map(int, open(0).read().split())

P = X[M - 1]

ave = sum(X) / N

v = (sum((x - ave) ** 2 for x in X) / N) ** 0.5
if v == 0:
    print(50)
else:
    print(int(50 + (P - ave) * 10 / v))