source_code: import java.math.*;
  import java.util.*;
  
  public class Main {
  	public static void main(String[] args) throws Exception {
  		Scanner scan = new Scanner(System.in);
  		long n = scan.nextLong();
  		long k = scan.nextLong();
  		long total = k * (k - 1) / 2 + 1;
  		if (total < n) {
  			System.out.println(-1);
  			return;
  		}
  		
  		long left = total - n;
  		long low = 1;
  		long high = k - 1;
  		while (low < high) {
  			long mid = (low + high) / 2;
  			long temp = mid * (mid + 1) / 2;
  			if (temp < left) {
  				low = mid + 1;
  			} else {
  				high = mid;
  			}
  		}
  		long temp = low * (low + 1) / 2;
  		if (temp == left) {
  			System.out.println(k - 1 - low);
  		} else {
  			System.out.println(k - low);
  		}
  	}
  }
  