public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int w = sc . nextInt ( ) ;
  int h = sc . nextInt ( ) ;
  int u1 = sc . nextInt ( ) ;
  int d1 = sc . nextInt ( ) ;
  int u2 = sc . nextInt ( ) ;
  int d2 = sc . nextInt ( ) ;
  int total = w ;
  for ( int i = h ;
  i > 0 ;
  i -- ) {
    total = total + i ;
    if ( i == d1 ) {
      total = total - u1 ;
    }
    if ( i == d2 ) {
      total = total - u2 ;
    }
    if ( total < 0 ) {
      total = 0 ;
    }
  }
  System . out . println ( total ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int w = sc . nextInt ( ) ; int h = sc . nextInt ( ) ; int u1 = sc . nextInt ( ) ; int d1 = sc . nextInt ( ) ; int u2 = sc . nextInt ( ) ; int d2 = sc . nextInt ( ) ; int total = w ; for ( int i = h ; i > 0 ; i -- ) { total = total + i ; if ( i == d1 ) { total = total - u1 ; } if ( i == d2 ) { total = total - u2 ; } if ( total < 0 ) { total = 0 ; } } System . out . println ( total ) ; }
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