import java.io.OutputStream; import java.io.IOException; import java.io.InputStream; import java.io.PrintWriter; import java.util.StringTokenizer; import java.io.IOException; import java.io.BufferedReader; import java.io.InputStreamReader; import java.io.InputStream; /** * Built using CHelper plug-in * Actual solution is at the top * * @author Chiaki.Hoshinomori */ public class Main { public static void main(String[] args) { InputStream inputStream = System.in; OutputStream outputStream = System.out; InputReader in = new InputReader(inputStream); PrintWriter out = new PrintWriter(outputStream); Task491 solver = new Task491(); solver.solve(1, in, out); out.close(); } static class Task491 { public void solve(int testNumber, InputReader in, PrintWriter out) { long n = in.nextLong(); n = n / 1000000001; if (n == 0) { out.println(0); } else { int ret = 0; char[] s = new Long(n).toString().toCharArray(); int[] pw = new int[10]; pw[0] = 1; for (int i = 1; i < 10; i++) { pw[i] = pw[i - 1] * 10; } for (int i = 1; i < s.length; ++i) { ret += 9 * pw[(i + 1) / 2 - 1]; } int now = 0; for (int i = 0; i < (s.length + 1) / 2; i++) { now = now * 10 + s[i] - '0'; int coef = s[i] - '0'; int rest = (s.length + 1) / 2 - (i + 1); if (i == 0) --coef; ret += coef * pw[rest]; } now *= pw[s.length - (s.length + 1) / 2]; for (int i = 0; i < s.length - (s.length + 1) / 2; ++i) { now += pw[i] * (s[i] - '0'); } if (now <= n) ++ret; out.println(ret); } } } static class InputReader { public BufferedReader reader; public StringTokenizer tokenizer; public InputReader(InputStream stream) { reader = new BufferedReader(new InputStreamReader(stream), 32768); tokenizer = null; } public String next() { while (tokenizer == null || !tokenizer.hasMoreTokens()) { try { tokenizer = new StringTokenizer(reader.readLine()); } catch (IOException e) { throw new RuntimeException(e); } } return tokenizer.nextToken(); } public long nextLong() { return Long.parseLong(next()); } } }