source_code: import java.io.InputStreamReader;
  import java.io.IOException;
  import java.util.ArrayList;
  import java.util.Arrays;
  import java.util.PriorityQueue;
  import java.util.Scanner;
  import java.io.BufferedReader;
  import java.io.OutputStream;
  import java.io.PrintWriter;
  import java.util.StringTokenizer;
  import java.io.InputStream;
  
  public class palin {
  	public static void main(String[] args) {
  		InputStream inputStream = System.in;
  		OutputStream outputStream = System.out;
  		InputReader in = new InputReader(inputStream);
  		PrintWriter out = new PrintWriter(System.out);
  		Scanner scan = new Scanner(System.in);
  		TaskC solver = new TaskC();
  		solver.solve(1, in, out);
  		out.close();
  	}
  }
  
  class TaskC {
  	public void solve(int testNumber, InputReader in, PrintWriter out) {
  		long n = in.nextLong() - 1;
  		long k = in.nextLong() - 1;
  		if (n == 0) {
  			out.print("0");
  		} else {
  			if (k >= n) {
  				out.print("1");
  			} else {
  				if (k * (k + 1) / 2 < n) {
  					out.print("-1");
  				} else {
  					long t = binsearch(n, k, 1, k);
  					long ans = k - t + 1;
  					if (k * (k + 1) / 2 - t * (t - 1) / 2 != n)
  						ans++;
  					System.out.println(ans);
  				}
  			}
  		}
  	}
  
  	public static long binsearch(long n, long k, long from, long to) {
  		if (from == to) {
  			return from;
  		}
  		long mid = (from + to) / 2;
  		if ((k * (k + 1)) / 2 - (mid * (mid - 1)) / 2 > n)
  			return binsearch(n, k, mid + 1, to);
  		else
  
  			return binsearch(n, k, from, mid);
  	}
  }
  
  class InputReader {
  	BufferedReader br;
  	StringTokenizer st;
  
  	public InputReader(InputStream in) {
  		br = new BufferedReader(new InputStreamReader(in));
  		st = null;
  	}
  
  	public String next() {
  		while (st == null || !st.hasMoreTokens()) {
  			try {
  				st = new StringTokenizer(br.readLine());
  			} catch (IOException e) {
  				throw new RuntimeException(e);
  			}
  		}
  		return st.nextToken();
  	}
  
  	public int nextInt() {
  		return Integer.parseInt(next());
  	}
  
  	public long nextLong() {
  		return Long.parseLong(next());
  	}
  }