public static void main ( String [ ] args ) {
  Scanner s = new Scanner ( System . in ) ;
  int n = s . nextInt ( ) ;
  int m = s . nextInt ( ) ;
  int minCorrect = 100 ;
  int maxCorrect = 0 ;
  int wrong = 100 ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    int a = s . nextInt ( ) ;
    if ( a < minCorrect ) {
      minCorrect = a ;
    }
    if ( a > maxCorrect ) {
      maxCorrect = a ;
    }
  }
  for ( int i = 0 ;
  i < m ;
  i ++ ) {
    int a = s . nextInt ( ) ;
    if ( a < wrong ) {
      wrong = a ;
    }
  }
  int correct = Math . max ( minCorrect * 2 , maxCorrect ) ;
  int answer ;
  if ( correct >= wrong ) {
    answer = - 1 ;
  }
  else {
    answer = correct ;
  }
  System . out . println ( answer ) ;
}

public static void main ( String [ ] args ) { Scanner s = new Scanner ( System . in ) ; int n = s . nextInt ( ) ; int m = s . nextInt ( ) ; int minCorrect = 100 ; int maxCorrect = 0 ; int wrong = 100 ; for ( int i = 0 ; i < n ; i ++ ) { int a = s . nextInt ( ) ; if ( a < minCorrect ) { minCorrect = a ; } if ( a > maxCorrect ) { maxCorrect = a ; } } for ( int i = 0 ; i < m ; i ++ ) { int a = s . nextInt ( ) ; if ( a < wrong ) { wrong = a ; } } int correct = Math . max ( minCorrect * 2 , maxCorrect ) ; int answer ; if ( correct >= wrong ) { answer = - 1 ; } else { answer = correct ; } System . out . println ( answer ) ; }
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