Add each digit from two positive integers and store the result in a String in the same order. If the sum of two digits exceeds 9 then put an asterisk in that digit's location in the String instead.

For example, 5366 and 5161 combine to become *4*7 because:

The two one's digits combine to equal 7 and become the first digit in the String, while the two ten's digits combine to be more than 9 so there is an asterisk instead of a number. The two numbers in the 100's place combine to equal 4 while the 1000's digits add up to 10 so there is an asterisk there instead.