source_code: import java.util.*;
  import java.math.*;
  public class B2 {
  
      /**
       * @param args
       */
      public static void main(String[] args) {
          // TODO Auto-generated method stub
          Scanner scan = new Scanner(System.in);
          BigInteger n = new BigInteger(scan.next());
          BigInteger k = new BigInteger(scan.next());
          BigInteger a = k.subtract(bi(1));
          BigInteger lim = k.multiply(a).divide(bi(2));
          lim = lim.add(bi(1));
          //System.out.println("lim: "+lim);
          if (n.compareTo(lim)>0){
              System.out.println(-1);
          }
          else {
              if (n.equals(1)){
                  System.out.println(0);
              }
              else {
                  BigInteger remain2 = lim.subtract(n).add(bi(1));
                  remain2 = remain2.multiply(bi(2));
                  //System.out.println(remain2);
                  double temp = remain2.doubleValue();
                  //System.out.println(temp);
                  long flr = (long)Math.sqrt(temp);
                  //System.out.println(flr);
                  BigInteger flr2 = bi(flr);
                  BigInteger rnd2 = remain2.subtract(flr2.multiply(flr2));
                  long rnd = remain2.longValue()-flr*flr;
                  //System.out.println("rnd "+rnd);
                  /*
                  if (rnd<=flr){
                      System.out.println(k.intValue()-flr);
                  }
                  else {
                      System.out.println(k.intValue()-(flr+1));
                  }
                  */
                  if (rnd2.compareTo(flr2)<=0){
                      System.out.println(k.subtract(flr2) );
                  }
                  else {
                      System.out.println(k.subtract(flr2.add(bi(1) ) ) );
                  }
              }
          }
      }
      public static BigInteger bi(int n1){
          return new BigInteger(""+n1);
      }
      public static BigInteger bi(long n1){
          return new BigInteger(""+n1);
      }
  
  }
  