public static void main ( String [ ] args ) {
  Scanner cin = new Scanner ( new BufferedInputStream ( System . in ) ) ;
  String a = cin . nextLine ( ) ;
  String b = cin . nextLine ( ) ;
  HashMap < Character , BigInteger > mp = new HashMap < > ( ) ;
  HashMap < BigInteger , Character > rmp = new HashMap < > ( ) ;
  String dic = new String ( "abcdefghijklmnopqrstuvwxyz" ) ;
  for ( int i = 0 ;
  i < 26 ;
  ++ i ) {
    mp . put ( dic . charAt ( i ) , BigInteger . valueOf ( i ) ) ;
    rmp . put ( BigInteger . valueOf ( i ) , dic . charAt ( i ) ) ;
  }
  BigInteger L = BigInteger . ONE ;
  BigInteger R = BigInteger . ONE ;
  for ( int i = 0 ;
  i < a . length ( ) ;
  ++ i ) {
    L = L . multiply ( BigInteger . valueOf ( 26 ) ) . add ( mp . get ( a . charAt ( i ) ) ) ;
    R = R . multiply ( BigInteger . valueOf ( 26 ) ) . add ( mp . get ( b . charAt ( i ) ) ) ;
  }
  if ( R . subtract ( L ) . equals ( BigInteger . ONE ) ) {
    System . out . println ( "No such string" ) ;
  }
  else {
    L = L . add ( BigInteger . ONE ) ;
    String ans = new String ( "" ) ;
    while ( ! L . equals ( BigInteger . ONE ) ) {
      ans += rmp . get ( L . mod ( BigInteger . valueOf ( 26 ) ) ) ;
      L = L . divide ( BigInteger . valueOf ( 26 ) ) ;
    }
    System . out . println ( new StringBuffer ( ans ) . reverse ( ) . toString ( ) ) ;
  }
}

public static void main ( String [ ] args ) { Scanner cin = new Scanner ( new BufferedInputStream ( System . in ) ) ; String a = cin . nextLine ( ) ; String b = cin . nextLine ( ) ; HashMap < Character , BigInteger > mp = new HashMap < > ( ) ; HashMap < BigInteger , Character > rmp = new HashMap < > ( ) ; String dic = new String ( " abcdefghijklmnopqrstuvwxyz " ) ; for ( int i = 0 ; i < 26 ; ++ i ) { mp . put ( dic . charAt ( i ) , BigInteger . valueOf ( i ) ) ; rmp . put ( BigInteger . valueOf ( i ) , dic . charAt ( i ) ) ; } BigInteger L = BigInteger . ONE ; BigInteger R = BigInteger . ONE ; for ( int i = 0 ; i < a . length ( ) ; ++ i ) { L = L . multiply ( BigInteger . valueOf ( 26 ) ) . add ( mp . get ( a . charAt ( i ) ) ) ; R = R . multiply ( BigInteger . valueOf ( 26 ) ) . add ( mp . get ( b . charAt ( i ) ) ) ; } if ( R . subtract ( L ) . equals ( BigInteger . ONE ) ) { System . out . println ( " No ▁ such ▁ string " ) ; } else { L = L . add ( BigInteger . ONE ) ; String ans = new String ( " " ) ; while ( ! L . equals ( BigInteger . ONE ) ) { ans += rmp . get ( L . mod ( BigInteger . valueOf ( 26 ) ) ) ; L = L . divide ( BigInteger . valueOf ( 26 ) ) ; } System . out . println ( new StringBuffer ( ans ) . reverse ( ) . toString ( ) ) ; } }
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