The convention is to use the error level of the program (i.e. the return value from it's main() function) as a diagnostic to tell if a program ran successfully or not. You should only return positive integers from 0 to 128 from your program, so as a calculator it's not very useful to use this value to return the result of your calculation.
Instead you should print the output of the calculation on stdout (i.e. use printf in your C program), and use the return value from the main function to indicate success or failure. e.g. return non-0 if there was an error, like the user not providing any numbers to add, or some parameter which is not a valid number.
Error messages should be printed to stderr. This was you can capture the correct output with the $(command) syntax (see below), error messages will be displayed on the terminal, and you will have the $? variable to check if there was an error.
Then you could call it like this from a bash script:
output=$(./add 1 11)
echo "the error level was $? ; the output from the program was: $output"