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 dy
   */
  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 pipes = 1;
          if(N == 1){
              out.println(0);return;
          }
          long left = 0, right = K - 1, find = -1;
          while (left <= right){
              long mid = (left + right) / 2;
              if(mid * mid + (1 - 2 * K) * mid + 2 * (N - 1) <= 0){
                  find = mid;
                  right = mid - 1;
              }
              else
                  left = mid + 1;
          }
          out.println(find);
      }
  }
  