Please answer a question about the following article about Dutch language:

Dutch is written using the Latin script. Dutch uses one additional character beyond the standard alphabet, the digraph IJ. It has a relatively high proportion of doubled letters, both vowels and consonants, due to the formation of compound words and also to the spelling devices for distinguishing the many vowel sounds in the Dutch language. An example of five consecutive doubled letters is the word voorraaddoos (food storage container). The diaeresis (Dutch: trema) is used to mark vowels that are pronounced separately when involving a pre- or suffix. Whereas a hyphen is used when this problem occurs in compound words. For example; "beïnvloed" (influenced), but zee-eend (sea duck). Generally, other diacritical marks only occur in loanwords, though the acute accent can also be used for emphasis or to differentiate between two forms. Its most common use is to differentiate between the indefinite article 'een' (a, an) and the numeral 'één' (one).

How many extra characters does Dutch have in addition to the standard alphabet?
one