import java.util.*;
public class champernowneverification_xiaowuc1 {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String have = sc.next();
    String want = "";
    for(int i = 1; i <= have.length(); i++) want += i;
    if(have.equals(want)) System.out.println(have.length());
    else System.out.println(-1);
  }
}