#include <iostream>
using namespace std;

int main() {
  int n, m, minus = 1;
  cin >> n;
  m = 1;
  for  (int a = 1; n> 0; a *= 10) {
    for  (int b = a; b < a*10; b++) {
      if ((n - minus) < 0) {
        cout << 0;
        return 0;
      }
      if ((n - minus) == 0) {
        cout << m;
        return 0;
      }
      n-= minus;
      m++;
    }
    minus++;
  }
  return 0;
}