source_code: import java.util.Scanner;
  import java.io.OutputStream;
  import java.io.IOException;
  import java.io.PrintWriter;
  import java.io.InputStream;
  
  /**
   * Built using CHelper plug-in
   * Actual solution is at the top
   */
  public class Main {
      public static void main(String[] args) {
          InputStream inputStream = System.in;
          OutputStream outputStream = System.out;
          Scanner in = new Scanner(inputStream);
          PrintWriter out = new PrintWriter(outputStream);
          TaskB solver = new TaskB();
          solver.solve(1, in, out);
          out.close();
      }
  }
  
  class TaskB {
      public void solve(int testNumber, Scanner in, PrintWriter out) {
          long n=in.nextLong();
          long k=in.nextInt();
          if(n==1){
              out.println(0);
              return;
          }
          long max=k*(k-1)/2+1;
          if(max<n){
              out.println("-1");
              return;
          }
  
          long low=1,high=k-1;
          long ans=k-1;
          while (low<=high){
              long mid=(low+high)/2;
              long val=mid*mid-mid*(2*k-1)+2*n-2;
              if(val>0){
                  low=mid+1;
              }
              else {
                  ans=mid;
                  high=mid-1;
              }
          }
          out.println(ans);
  
      }
  }
  
  