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
   * @author Saul
   */
  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() ;
      int k = in.nextInt();
      long top = 1L * k * (k+1) / 2L - k + 1;
  
      if ( n == 1L ){
        out.print(0);
        return;
      }
  
      if ( n > top ){
        out.print(-1);
        return;
      }
  
      int ans = 0;
      if ( n > 0 ){
        ++ans;
        n -= k;
        k -= 2;
      }
  
      while ( n > 0 ){
        ++ans;
        n -= k;
        k--;
      }
      out.print(ans);
    }
  }
  