import std.algorithm, std.conv, std.range, std.stdio, std.string;

void main()
{
  auto n = readln.chomp.to!long, k = (n/(10^^9+1)).to!int;

  auto s = k.to!string, ns = s.length;
  auto r = 0;
  foreach (i; 1..ns)
    r += 10^^((i+1)/2-1) * 9;

  auto t = s[0..(ns+1)/2];
  r += t.to!int;

  if (k < chain(s[0..(ns+1)/2], s[0..ns/2].retro).to!int) --r;

  writeln(r);
}