y = x = int(input())
k = 0
while y:
    k += 1
    y //= 10
z = (k + 1) // 2
number = 0
for i in  range(z, 0, -1):
    number += 10**i - 10**(i-1)
    f = 10 ** (i - 1)
    for j in range(1, 10):
        if number ** 2 <= x:
            break
        else:
            number -= f
print (number)