source_code: /*
   * To change this template, choose Tools | Templates
   * and open the template in the editor.
   */
  
  import java.util.Scanner;
  
  /**
   *
   * @author 111
   */
  public class JavaApplication4 {
  
      /**
       * @param args the command line arguments
       */
      static long k, n, ans;
      static private long binsearch(long l, long r)
      {
          if(l==r) return l;
          long m=(l+r)/2;
          long res=(m*(k+k-m+1)/2);
          if(res>=n)
              return binsearch(l, m);
          else
              return binsearch(m+1, r);
      }
      public static void main(String[] args) {
          Scanner in=new Scanner(System.in);
          n=in.nextLong();
          k=in.nextLong();
          n--;
          k--;
          if(k*(k+1)/2<n)
              ans=-1;
          else
              ans=binsearch(0, k);
          System.out.println(ans);            
      }
  }
  