The Simple Calculator can perform addition, subtraction, multiplication, and division. The program detects invalid user input and prompts the user to enter valid input.
Example:
**Case 1: Invalid number**
Enter a number: a
Error! It is NOT a number
**Case 2: Invalid operator**
Enter an operator: h
Error! Operator is not correct
**Case 3: Correct value**
Enter a number: 5
Enter an operator: +
Enter a number: 10
Result: 15.00