N = int(input())
result = 0
n = N // (10**9+1)
keta = len(str(n))
for i in range(keta-1):
    result += 9 * (10 ** (i // 2))
k = 0
h = (keta + 1) // 2
ki = False
if keta % 2 == 1:
    ki = True
mini = "1" + "0" * (h-1)
maxi = str(n)[:h]
result += int(maxi) - int(mini)
if ki:
    maximam = maxi
    for i in range(len(maxi)-1):
        maximam += maxi[-2-i]
else:
    maximam = maxi
    for i in range(len(maxi)):
        maximam += maxi[-1-i]
if n >= int(maximam):
    result += 1
print(result)