from math import sqrt, floor


def main():
    N = int(input())
    scores = list(map(int, input().split()))
    M = int(input())

    average = sum(scores) / N
    std_deviation = sqrt(
        sum(map(lambda score: (score - average)**2, scores))/N)
    try:
        deviation_score = 50 + (scores[M-1] - average)*10/std_deviation
    except ZeroDivisionError:
        deviation_score = 50

    if deviation_score >= 0:
        print(floor(deviation_score))
    else:
        print(-1*floor(abs(deviation_score)))


if __name__ == "__main__":
    main()