public static void main ( String [ ] args ) {
  Scanner scan = new Scanner ( System . in ) ;
  int w = scan . nextInt ( ) ;
  int h = scan . nextInt ( ) ;
  int u1 = scan . nextInt ( ) ;
  int d1 = scan . nextInt ( ) ;
  int u2 = scan . nextInt ( ) ;
  int d2 = scan . nextInt ( ) ;
  if ( d2 > d1 ) {
    d1 += d2 ;
    d2 = d1 - d2 ;
    d1 -= d2 ;
    u1 += u2 ;
    u2 = u1 - u2 ;
    u1 -= u2 ;
  }
  w += ( ( h * ( h + 1 ) / 2 ) ) - ( ( d1 - 1 ) * d1 / 2 ) ;
  w -= u1 ;
  if ( w < 0 ) w = 0 ;
  w += ( ( d1 * ( d1 - 1 ) ) / 2 ) - ( ( d2 - 1 ) * d2 / 2 ) ;
  w -= u2 ;
  if ( w < 0 ) w = 0 ;
  w += ( d2 * ( d2 - 1 ) ) / 2 ;
  System . out . println ( w ) ;
}

public static void main ( String [ ] args ) { Scanner scan = new Scanner ( System . in ) ; int w = scan . nextInt ( ) ; int h = scan . nextInt ( ) ; int u1 = scan . nextInt ( ) ; int d1 = scan . nextInt ( ) ; int u2 = scan . nextInt ( ) ; int d2 = scan . nextInt ( ) ; if ( d2 > d1 ) { d1 += d2 ; d2 = d1 - d2 ; d1 -= d2 ; u1 += u2 ; u2 = u1 - u2 ; u1 -= u2 ; } w += ( ( h * ( h + 1 ) / 2 ) ) - ( ( d1 - 1 ) * d1 / 2 ) ; w -= u1 ; if ( w < 0 ) w = 0 ; w += ( ( d1 * ( d1 - 1 ) ) / 2 ) - ( ( d2 - 1 ) * d2 / 2 ) ; w -= u2 ; if ( w < 0 ) w = 0 ; w += ( d2 * ( d2 - 1 ) ) / 2 ; System . out . println ( w ) ; }
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