source_code: import java.io.BufferedReader;
  import java.io.InputStreamReader;
  import java.util.Scanner;
  import java.util.Vector;
  
  /**
   * Created with IntelliJ IDEA.
   * User: horikawa
   * Date: 3/23/13
   * Time: 1:29 AM
   * To change this template use File | Settings | File Templates.
   */
  public class B {
      public static void main (String[] argv) {
          Scanner in = new Scanner(System.in);
          long n = in.nextLong();
          long k = in.nextLong();
          long max = ((k*(k-1))/2L)+1L;
          long ans = -1;
          if (n == 1) {
              System.out.println(0);
              return;
          }
          if (max < n) {
              ans = -1;
          } else if (max == n) {
              ans = k-1;
          } else {
              if (k >= n) {
                  ans = 1;
              } else {
                  long low = 1;
                  long high = k-1;
                  while (high > low+1) {
                      long mid = (low+high)/2;
                      long sum = (((mid+(k-1)) * (k-mid)) / 2) + 1;
                      if (sum >= n) {
                          low = mid;
                      } else {
                          high = mid;
                      }
                  }
                  ans = (k - low);
              }
          }
          System.out.println(ans);
          return;
      }
  }
  