public static void main ( String [ ] args ) {
  Scanner in = new Scanner ( System . in ) ;
  String s = in . next ( ) ;
  int a4 = 0 , a7 = 0 , i ;
  in . close ( ) ;
  for ( i = 0 ;
  i < s . length ( ) ;
  i ++ ) {
    if ( s . charAt ( i ) == '4' ) a4 ++ ;
    if ( s . charAt ( i ) == '7' ) a7 ++ ;
  }
  System . out . println ( a4 + a7 > 0 ? a4 >= a7 ? 4 : 7 : - 1 ) ;
}

public static void main ( String [ ] args ) { Scanner in = new Scanner ( System . in ) ; String s = in . next ( ) ; int a4 = 0 , a7 = 0 , i ; in . close ( ) ; for ( i = 0 ; i < s . length ( ) ; i ++ ) { if ( s . charAt ( i ) == '4' ) a4 ++ ; if ( s . charAt ( i ) == '7' ) a7 ++ ; } System . out . println ( a4 + a7 > 0 ? a4 >= a7 ? 4 : 7 : - 1 ) ; }
(method_declaration (modifiers) type: (void_type) name: (identifier) parameters: (formal_parameters (formal_parameter type: (array_type element: (type_identifier) dimensions: (dimensions)) name: (identifier))) body: (block (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list (field_access object: (identifier) field: (identifier)))))) (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal)) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal)) declarator: (variable_declarator name: (identifier))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))) (for_statement init: (assignment_expression left: (identifier) right: (decimal_integer_literal)) condition: (binary_expression left: (identifier) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))) update: (update_expression (identifier)) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))) right: (character_literal))) consequence: (expression_statement (update_expression (identifier)))) (if_statement condition: (parenthesized_expression (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))) right: (character_literal))) consequence: (expression_statement (update_expression (identifier)))))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (ternary_expression condition: (binary_expression left: (binary_expression left: (identifier) right: (identifier)) right: (decimal_integer_literal)) consequence: (ternary_expression condition: (binary_expression left: (identifier) right: (identifier)) consequence: (decimal_integer_literal) alternative: (decimal_integer_literal)) alternative: (unary_expression operand: (decimal_integer_literal))))))))