Please read the following table in HTML format from a webpage and then answer the question in detail according to the table. Each table cell is defined by a <td> and a </td> tag. Each table row starts with a <tr> and ends with a </tr> tag. 'rowspan=m' and 'colspan=n' represents a cell spaning over 'm' rows and 'n' columns.

Webpage title:
Zala Kralj & Gašper Šantl
Table title:
Singles
Table:
<table id='hittask' contenteditable='False'>
<tr> <th colspan=1 rowspan=2 > Title </th><th colspan=1 rowspan=2 > Year </th><th colspan=1 rowspan=1 > Peak chart positions </th><th colspan=1 rowspan=2 > Album </th></tr>
<tr> <th colspan=1 rowspan=1 > SLO </th></tr>
<tr> <th class="highlighted"  colspan=1 rowspan=1 > "Valovi" </th><td class="highlighted"  colspan=1 rowspan=3 > 2018 </td><td colspan=1 rowspan=1 > — </td><td class="highlighted"  colspan=1 rowspan=4 > Štiri </td></tr>
<tr> <th class="highlighted"  colspan=1 rowspan=1 > "Baloni" </th><td colspan=1 rowspan=1 > — </td></tr>
<tr> <th class="highlighted"  colspan=1 rowspan=1 > "S teboi" </th><td colspan=1 rowspan=1 > — </td></tr>
<tr> <th colspan=1 rowspan=1 > "Sebi" </th><td class="highlighted"  colspan=1 rowspan=1 > 2019 </td><td colspan=1 rowspan=1 > 9 </td></tr>
<tr> <td colspan=22 rowspan=1 > "—" denotes a single that did not chart or was not released. </td></tr>
</table>
Question:
What was  Zala Kralj & Gašper Šantl highlights in the year 2018 and 2019?
Answer:
Zala Kralj & Gašper Šantl released the singles "Valovi", "Baloni", and "S teboi" in 2018, before releasing their debut extended play Štiri in 2019.