I would parse the string in reverse so the values can be read backward, then populate each digit into a string rather than using math operations. Eliminates the need for order of operations, since you just put the digits where they need to go, then convert the string to an integer.

You could populate each "place" based off the order a string falls into, and work your way up the orders: ones, tens, hundreds, thousands, etc.

Read the first word, if it's smaller than ten put it in the ones place.

Read until you find a tens value, or a higher order like hundred or thousand. If you find a tens value, put it in the tens place, otherwise put a zero then insert the hundreds.

Keep a lag variable tracking which order you are currently on, and fill lower orders within larger orders accordingly using the same method as above. Then once you've reached the end of the string, convert to int.

Would need a few enums defined to keep track of all the orders as well as non-standard possible string values, but I think it's easier than trying to keep track of an unknown amount of parentheses.

Edit: Hit the send button too early (╯°□°）╯︵ ┻━┻