import sys
from itertools import accumulate
sys.setrecursionlimit(10**7)
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int,sys.stdin.readline().rstrip().split())
def LI(): return list(map(int,sys.stdin.readline().rstrip().split()))
def LI2(): return list(map(int,sys.stdin.readline().rstrip()))
def S(): return sys.stdin.readline().rstrip()
def LS(): return list(sys.stdin.readline().rstrip().split())
def LS2(): return list(sys.stdin.readline().rstrip())


N = I()

M = N//(10**9+7)
# M以下の正の整数の中で、回文が何個あるかを答えれば良い

if M == 0:
    print(0)
    exit()

X = [0,9,9,90,90,900,900,9000,9000]
AX = list(accumulate(X))
# AX[i] = i桁以下の回文の個数

d = len(str(M))  # Mの桁数
ans = AX[d-1]

B = M//(10**(d//2))
ans += B-10**((d-1)//2)

C = str(B)
for i in range(d//2-1,-1,-1):
    C += str(C[i])
C = int(C)

if C <= M:
    ans += 1

print(ans)