source_code: import java.util.*;
  import java.lang.*;
  import java.io.BufferedReader;
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.InputStreamReader;
  import java.io.*;
   
  public class Main {
      
      public static void main(String[] args) throws Exception{
          FastReader sc=new FastReader(); 
          OutputStream outputStream = System.out;
          PrintWriter out = new PrintWriter(outputStream);
          Main mm=new Main();
          long n=sc.nextLong();
          long k=sc.nextLong();
          long l=0;
          long r=1000000000;
          long ans=-1;
          while(l<=r) {
              long mid=(l+r)/2;
              if(n-mid<=0) {
                  r=mid-1;
              }
              else {
                  long temp=(n-mid)*(n-mid+1)-(2*mid);
                  if(temp==2*k) {
                      ans=mid;
                      break;
                  }
                  else if(temp<2*k) {
                      r=mid-1;
                  }
                  else if(temp>2*k) {
                      l=mid+1;
                  }
              }
          }
          System.out.println(ans);
      }  
  }
  
  
  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; 
      } 
  } 