n = int(input())
x = list(map(int,input().split()))
m = int(input())
ave = sum(x)/len(x)
x1 = [(z-ave)**2 for z in x]
r = (sum(x1)/len(x1))**0.5
if r != 0:
    s = (x[m-1]-ave)* 10 / r
    if x[m-1] > ave:
        s = s + 50
    else:
        s = 50 - s
else:
    s = 50
print(int(s))