You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Serbo-Croatian

Context:
The jat-reflex rules are not without exception. For example, when short jat is preceded by r, in most Ijekavian dialects developed into /re/ or, occasionally, /ri/. The prefix prě- ("trans-, over-") when long became pre- in eastern Ijekavian dialects but to prije- in western dialects; in Ikavian pronunciation, it also evolved into pre- or prije- due to potential ambiguity with pri- ("approach, come close to"). For verbs that had -ěti in their infinitive, the past participle ending -ěl evolved into -io in Ijekavian Neoštokavian.

Question:
To what did the ending -ei evolve into in Ijekavian Neostokavian?

-io