TheNumberOnBoard.java

/** https://codeforces.com/problemset/problem/835/B #greedy */
import java.util.Arrays;
import java.util.Scanner;

class TheNumberOnBoard {
  public static void main(String[] args) {
    // input
    Scanner sc = new Scanner(System.in);
    int k = sc.nextInt();
    String n = sc.next();
    char[] charArr = n.toCharArray();

    // calculate
    int sumDigits = 0;
    for (char c : charArr) {
      sumDigits += c - '0';
    }
    if (sumDigits >= k) {
      System.out.println(0);
      return;
    }
    // iterate from the minimum digit
    Arrays.sort(charArr);
    int ret = 0;
    for (char c : charArr) {
      sumDigits += '9' - c;
      ret++;
      if (sumDigits >= k) {
        break;
      }
    }
    System.out.println(ret);
  }
}