Check for valid integer. Add two digits with carry. Add two numbers. Subtract two numbers. Convert a string's digits from 3D to 0D where D is a decimal digit Preprocess a string for subtraction by adjusting for borows.
Many enhancements are possible.
Here are a few. Handle floating point numbers in one or more formats, for ex. Implement other mathematical functions such as trig functions or ceiling or floo, etc. You should create two procedures to do the conversions. For the base program you need not check for valid integers.