I can't figure out how to write the code for the error section. How do you test if the input is illegal? The input is being read from the int main() function

The definition of VERIFY_CUSTOMER method is as follows: g.VERIFY_CUSTOMER(x, y); where g is a object of class SIMPLER PHONE_BILL, and x and y are integers This method should check if there is a customer whose phone number isx and pin is y. If so, it prints the following into out.1: OUTPUT FROM VERIFY_CUSTOMER INTERFACE: THERE IS A CUSTOMER WITH x AND PIN y END If there is no such customer, the output to out.1 is: OUTPUT FROM VERIFY_CUSTOMER INTERFACE: NO SUCH CUSTOMER. If x and/or y is illegal, the output to out.1 is: OUTPUT FROM VERIFY_CUSTOMER INTERFACE: INPUT ERROR END MAKE SURE THAT EVERY LINE TO out.1 ENDS WITH endl