source_code: import java.util.Scanner;
  
  public class B{
      public static void main(String args[]){
          Scanner input = new Scanner(System.in);
          long n = input.nextLong();
          long k = input.nextLong();
          System.out.println(solve(n, k));
          input.close();
      }
      
      public static long solve(long n, long k){
          long dis = n - k;
          if(n == 1)
              return 0;
          if((((k - 2) * ((k - 2) + 1)) / 2) + 1 <= dis)
              return -1;
          if(k >= n)
              return 1;
          //
          long ans = 2;
          long now = (((k - 2) * ((k - 2) + 1)) / 2) + 1 + k;
          long dist = Math.abs(now - n);
          long delta = 1 + 8 * dist;
          double ret = (1 + Math.sqrt(delta)) / 2;
          //
          now = (((k - 2) * ((k - 2) + 1)) / 2) + 1 + k;
          dist = Math.abs(now - k);
          delta = 1 + 8 * dist;
          double nret = (1 + Math.sqrt(delta)) / 2;
          //
          double back = nret - ret;
          ans = (long) back;
          return ans + 2;
      }
  }