public static void main ( String args [ ] ) throws Exception {
  StringTokenizer stok = new StringTokenizer ( new Scanner ( System . in ) . useDelimiter ( "\\A" ) . next ( ) ) ;
  StringBuilder sb = new StringBuilder ( ) ;
  int n = Integer . parseInt ( stok . nextToken ( ) ) ;
  long [ ] a = new long [ n + 2 ] ;
  for ( int i = 1 ;
  i <= n ;
  i ++ ) a [ i ] = Long . parseLong ( stok . nextToken ( ) ) ;
  long [ ] l = new long [ n + 2 ] ;
  long [ ] r = new long [ n + 2 ] ;
  for ( int i = 1 ;
  i <= n ;
  i ++ ) l [ i ] = Math . min ( l [ i - 1 ] + 1 , a [ i ] ) ;
  for ( int i = n ;
  i >= 1 ;
  i -- ) r [ i ] = Math . min ( r [ i + 1 ] + 1 , a [ i ] ) ;
  for ( int i = 1 ;
  i <= n ;
  i ++ ) l [ i ] = Math . min ( l [ i ] , r [ i ] ) ;
  System . out . println ( Arrays . stream ( l ) . max ( ) . getAsLong ( ) ) ;
}

public static void main ( String args [ ] ) throws Exception { StringTokenizer stok = new StringTokenizer ( new Scanner ( System . in ) . useDelimiter ( " \\ A " ) . next ( ) ) ; StringBuilder sb = new StringBuilder ( ) ; int n = Integer . parseInt ( stok . nextToken ( ) ) ; long [ ] a = new long [ n + 2 ] ; for ( int i = 1 ; i <= n ; i ++ ) a [ i ] = Long . parseLong ( stok . nextToken ( ) ) ; long [ ] l = new long [ n + 2 ] ; long [ ] r = new long [ n + 2 ] ; for ( int i = 1 ; i <= n ; i ++ ) l [ i ] = Math . min ( l [ i - 1 ] + 1 , a [ i ] ) ; for ( int i = n ; i >= 1 ; i -- ) r [ i ] = Math . min ( r [ i + 1 ] + 1 , a [ i ] ) ; for ( int i = 1 ; i <= n ; i ++ ) l [ i ] = Math . min ( l [ i ] , r [ i ] ) ; System . out . println ( Arrays . stream ( l ) . max ( ) . getAsLong ( ) ) ; }
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name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))))) (local_variable_declaration type: (array_type element: (integral_type) dimensions: (dimensions)) declarator: (variable_declarator name: (identifier) value: (array_creation_expression type: (integral_type) dimensions: (dimensions_expr (binary_expression left: (identifier) right: (decimal_integer_literal)))))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (identifier)) update: (update_expression (identifier)) body: (expression_statement (assignment_expression left: (array_access array: (identifier) index: (identifier)) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))))))) (local_variable_declaration type: (array_type element: (integral_type) dimensions: (dimensions)) declarator: (variable_declarator name: (identifier) value: (array_creation_expression type: (integral_type) dimensions: (dimensions_expr (binary_expression left: (identifier) right: (decimal_integer_literal)))))) (local_variable_declaration type: (array_type element: (integral_type) dimensions: (dimensions)) declarator: (variable_declarator name: (identifier) value: (array_creation_expression type: (integral_type) dimensions: (dimensions_expr (binary_expression left: (identifier) right: (decimal_integer_literal)))))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (identifier)) update: (update_expression (identifier)) body: (expression_statement (assignment_expression left: (array_access array: (identifier) index: (identifier)) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (binary_expression left: (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal))) right: (decimal_integer_literal)) (array_access array: (identifier) index: (identifier))))))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (identifier))) condition: (binary_expression left: (identifier) right: (decimal_integer_literal)) update: (update_expression (identifier)) body: (expression_statement (assignment_expression left: (array_access array: (identifier) index: (identifier)) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (binary_expression left: (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal))) right: (decimal_integer_literal)) (array_access array: (identifier) index: (identifier))))))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (identifier)) update: (update_expression (identifier)) body: (expression_statement (assignment_expression left: (array_access array: (identifier) index: (identifier)) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (array_access array: (identifier) index: (identifier)) (array_access array: (identifier) index: (identifier))))))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (method_invocation object: (method_invocation object: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))) name: (identifier) arguments: (argument_list)) name: (identifier) arguments: (argument_list)))))))