import numpy as np


def main():
    _ = int(input())
    x = np.array(list(map(int, input().split())))
    m = int(input())

    if np.std(x) == 0:
        ans = 50
    else:
        ans = int(50 + 10 * (x[m - 1] - np.average(x)) / np.std(x))
    print(ans)


if __name__ == "__main__":
    main()