N = int(input())
X = list(map(int, input().split()))
A = []
M = 0
for i in range(N):
    A.append(X[i])
    M += A[i]
M /= N
AS = True
for i in range(1, N):
    if A[i] != A[0]:
        AS = False
        break
if AS:
    print("50")
    exit(0)
Q = 0
for i in range(N):
    Q += (A[i] - M) ** 2
RQ = (Q / N) ** 0.5
M1 = int(input())
ret = 50 + 10 * (A[M1 - 1] - M) / RQ
if 50 + 10 * (A[M1 - 1] - M) / RQ >= 0:
    print(int(50 + 10 * (A[M1 - 1] - M) / RQ))
else:
    print(-int(-50 + 10 * (A[M1 - 1] - M) / RQ))