Code:
do
			{
				// display title
				printf("Individual Salesman Commission\n");
				printf("==============================\n");


				// show tip for first input
				if (*tip == 1)
				{
					// display tip 
					printf("Tip: Just input the number instead of the whole id.\n\n");
				}


                // prompt and read salesman id
                printf("Salesman ID (NO.): ");
                
				// if it is not a valid input 
                if (scanf("%d",&stop) == NULL)
                {
                    valid = 0; // continue loop process
                    printf("Invalid input. Please input again.\n\n");
					//system("pause"); // freeze display message
					printf("\nPress Enter to continue...\n"); 
					getchar();
					getchar(); // prompt user
                }
                else
                    valid = 1; // end loop process


				// clear input buffer
				while ((ch = getchar()) != '\n' && ch != EOF);


				// clear screen display  
				system("cls");
            } while (valid == 0); // loop if the input is invalid
1) Why it straight away end the loop after I used getchar() rather than system("pause")?
2) How can I validate number+character/string such as 1asd?