N = int(input())
X = list(map(int, input().split()))
M = int(input())

Ans = 0

# 平均点
A = sum(X) / N
# print(A)

# 全員の平均点との差の平方
X2 = []
for i in X:
    X2 += [(i-A)**2]
# print(X2)

# その平均の平方根
A2 = (sum(X2) / N)**0.5
# print(A2)

if A2 == 0:
    Ans = 50
else:
    A3 = (abs(A - X[M-1]) * 10) / A2

    if X[M-1] > A:
        Ans = 50 + A3
    else:
        Ans = 50 - A3

print(int(Ans))