n = input().strip()
k = len(n)
ans = 0

# Count palindromes for all lengths shorter than k
for i in range(1, k):
    ans += 9 * (10 ** ((i - 1) // 2))

m = (k + 1) // 2  # ceil(k/2)
prefix = n[:m]

# Construct the palindrome from the prefix
if k % 2 == 0:
    constructed = prefix + prefix[::-1]
else:
    constructed = prefix + prefix[:-1][::-1]

# Determine the maximum valid prefix
if int(constructed) > int(n):
    max_p = int(prefix) - 1
else:
    max_p = int(prefix)

min_p = 10 ** (m - 1)

# Calculate the count of valid prefixes and add to the answer
count = max(0, max_p - min_p + 1) if max_p >= min_p else 0
ans += count

print(ans)