public static void main ( String [ ] args ) {
  Scanner cin = new Scanner ( System . in ) ;
  int caseN = cin . nextInt ( ) ;
  for ( int caseI = 1 ;
  caseI <= caseN ;
  caseI ++ ) {
    int n = cin . nextInt ( ) ;
    BigInteger [ ] times = new BigInteger [ n ] ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) times [ i ] = new BigInteger ( cin . next ( ) ) ;
    BigInteger ans = times [ 0 ] . subtract ( times [ 1 ] ) . abs ( ) ;
    for ( int i = 2 ;
    i < n ;
    i ++ ) ans = ans . gcd ( times [ i ] . subtract ( times [ i - 1 ] ) . abs ( ) ) ;
    ans = ans . subtract ( times [ 0 ] . mod ( ans ) ) . mod ( ans ) ;
    System . out . printf ( "Case #%d: %s" , caseI , ans . toString ( ) ) ;
    System . out . println ( ) ;
  }
}

public static void main ( String [ ] args ) { Scanner cin = new Scanner ( System . in ) ; int caseN = cin . nextInt ( ) ; for ( int caseI = 1 ; caseI <= caseN ; caseI ++ ) { int n = cin . nextInt ( ) ; BigInteger [ ] times = new BigInteger [ n ] ; for ( int i = 0 ; i < n ; i ++ ) times [ i ] = new BigInteger ( cin . next ( ) ) ; BigInteger ans = times [ 0 ] . subtract ( times [ 1 ] ) . abs ( ) ; for ( int i = 2 ; i < n ; i ++ ) ans = ans . gcd ( times [ i ] . subtract ( times [ i - 1 ] ) . abs ( ) ) ; ans = ans . subtract ( times [ 0 ] . mod ( ans ) ) . mod ( ans ) ; System . out . printf ( " Case ▁ # % d : ▁ % s " , caseI , ans . toString ( ) ) ; System . out . println ( ) ; } }
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