making a slide show with one audio file per file, using ffmpeg

I have a series of images and sound files with matching filenames:

1.png, 1.wav
2.png, 2.wav
...
20.png, 20.wav
etc...

I would like to make a video where each image appears for the duration of its sound file.

I can make a video from one image / one sound file:

ffmpeg -loop 1 -i 1.png -i 1.wav -c:v libx264 -tune stillimage -c:a aac -b:a 192k -pix_fmt yuv420p -shortest output.mp4

but how do I make that with a series?

I tried to use:

-i %d.png -i %d.wav

but I get an error that it can’t find the audio file.

or, should I make a bunch of small videos and stitch them together?