import sys

def II(): return int(sys.stdin.readline())

def solve():
    n=II()
    ans=0
    for k in range(1,10):
        h=(k+1)//2
        for l in range(10**(h-1),10**h):
            s=str(l)
            a=int(s+s[::-1][k&1:])*1000000001
            if a>n:return ans
            ans+=1
    return ans

print(solve())