180 bottles of soda can be replaced with 36 bottles of soda, and these 36 bottles can be replaced with 7 bottles of soda, leaving an empty bottle.

Seven bottles of soda can be replaced by 1 bottle, leaving two empty bottles. Finally, 180 bottles of soda can drink up to 180+36+7+ 1=224 bottles of soda.

There are still four empty bottles and 22 soda bottles are missing.

If you buy another bottle of soda, you can change those four empty bottles into 1 flat soda, so there are 20 bottles of soda missing, leaving one empty bottle.

I bought four more bottles of soda, and after drinking them, I added an empty bottle left over from last time, so I can change it to 1 bottle of soda, and I still need 15 bottles of soda, leaving one empty bottle.

It can be seen that every time you buy 4 bottles of soda, you can reduce the demand for 5 bottles, so if you buy less 15 bottles, you need to buy 12 bottles of soda.

In general, you need to buy 1+4+ 12= 17 bottles of soda.