Fancy watching an opera in a Roman amphitheatre ?

Avenches are running Aida from the 6th to 21st July .... tickets 60-170 chuffs (cheap for opera)

Saw Radiohead at this venue last year & is pretty cool. For those that don't know it, Avenches is in Kanton Vaud & has one of the best preserved amphitheatres in N Europe, as it was the Roman capitl of Helvetia Province - www.avenches.ch

The website says that you can get tickets for 40CHF (sector I) on the evening of the performance.
What are the chances of getting them ?

Personally, I wouldn't leave it to chance as the amphitheatre is not that big & fills quickly.