source_code: import java.util.*;
  import java.io.*;
  
  public class Main
  {
  	public static void main(String[] args)
  	{
  		FastReader reader = new FastReader();
  		PrintWriter writer = new PrintWriter(System.out);
  
  		long n = reader.nextLong();
  		long k = reader.nextLong();
  
  		long s=0;
  		long e=n;
  
  		long ans = -1;
  		while (s<=e) {
  			long m = (s+e)/2;
  			long temp = ((n-m)*(n-m+1))/2 - m;
  
  			if (temp < k) 
  				e = m-1;
  			else if (temp > k)
  				s = m+1;
  			else {
  				ans = m;
  				break;
  			} 
  		}
  
  		writer.println(ans);
  		writer.close();
  	}
  
  	static class FastReader
  	{
  		BufferedReader br;
  		StringTokenizer st;
  
  		public FastReader()
  		{
  			br = new BufferedReader(new InputStreamReader(System.in));
  		}
  
  		String next()
  		{
  			while (st == null || !st.hasMoreElements())
  			{
  				try
  				{
  					st = new StringTokenizer(br.readLine());
  				}
  				catch (IOException e)
  				{
  					e.printStackTrace();
  				}
  			}
  			return st.nextToken();
  		}
  
  		int nextInt()
  		{
  			return Integer.parseInt(next());
  		}
  
  		long nextLong()
  		{
  			return Long.parseLong(next());
  		}
  
  		double nextDouble()
  		{
  			return Double.parseDouble(next());
  		}
  
  		String nextLine()
  		{
  			String str = "";
  			try
  			{
  				str = br.readLine();
  			}
  			catch (IOException e)
  			{
  				e.printStackTrace();
  			}
  			return str;
  		}
  	}
  }