source_code: import java.io.*;
  import java.util.*;
  
  public class Main
  {
  	long sum ( long n ) { return (n*(n+1))/2; }
  
  	public void solve ( ) throws Exception
  	{
  		Scanner in = new Scanner ( System.in );
  		long n = in.nextLong()-1;
  		long k = in.nextLong()-1;
  		
  		long lo = 0, hi = k, mi;
  		while ( lo < hi )
  		{
  			mi = ( lo + hi ) / 2;
  			if ( sum(k)-sum(k-mi-1) <= n ) lo = mi+1;
  			else hi = mi;
  		}
  		
  		long ans = lo;
  		n -= ( sum(k) - sum(k-ans) );
  		k -= ans;
  
  		if ( n > k ) println ( "-1" );
  		else if ( n == 0 ) println ( ans );
  		else println ( (ans+1) );
  	}
  	
  	public static void main ( String[] args ) throws Exception { (new Main()).solve(); }
  	public static void print ( Object o ) { System.out.print ( o ); }
  	public static void println ( Object o ) { System.out.println ( o ); }
  	public static void println ( ) { System.out.println ( ); }
  }
  