Look through here on how to solve CI by hand. Once you know by hand you can do it in code.
Look up things you don't know how to do like getting input, displaying in output buffer, using the binary operators (+-/*) and their order of operation.
This is very critical for beginners to do because you wont have help all the time.
In conclusion, do it by hand first then convert to code. research what you don't know how to do