Compare the operator to '+'. If equal, add the two values. If not equal, multiply the two values. Print the result.
Let me fill in some gaps. You will need a loop to convert the characters to numbers
If the first character is '-' set the value to -1 otherwise set it to 1.
a) Subtract '0' from each character and multiply the result by that.
If there are more characters, multiple the result by 10 and goto a).