source_code: import java.io.*;
  import java.util.*;
  
  public class B
  {
  	public static void main(String [] args) throws IOException
  	{
  		Scanner in = new Scanner(System.in);
  
  		long n = in.nextLong();
  		long k = in.nextLong();
  		if(n == 1)
  		{
  			System.out.println(0);
  			return;
  		}
  		if(n <= k)
  		{
  			System.out.println(1);
  			return;
  		}
  
  		long lb = 2, ub = k;
  		long sum = ((k)*(k-1))/2;
  		if(sum+1 < n)
  		{
  			System.out.println(-1);
  			return;
  		}
  		while(ub - lb > 1)
  		{
  			long mid = (lb+ub)/2;
  			long s = ((mid-1)*(mid-2))/2;
  			if(n - (sum-s+1) < 0)
  				lb = mid;
  			else
  				ub = mid;
  		}
  
  		long rem = n - (sum - ((ub-1)*(ub-2))/2 + 1);
  		long res = k - ub + 1;
  		if(rem == 0)
  		{
  			System.out.println(res);
  			return;
  		}
  		rem++;
  		if(!(rem >= 2 && rem < ub))
  		{
  			System.out.println(-1);
  			return;
  		}
  		System.out.println(res+1);
  
  
  	}
  }