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
result += (int(str(n)[0]) - 1) * (10 ** (h - 1))
max = str(n)[0]
for i in range(1, h):
    result += int(str(n)[i]) * (h - i - 1)
    max += str(n)[i]
for i in range(h):
    if i == 1 & keta % 2 == 1:
        continue
    max += max[h - 1 - i]
if int(max) < n:
    result += 1
print(result)