C :: Calculator - Input Specific Number?

In my calculator, I am trying to make it so that you put enter 1 to add, 2 to subtract, 3 to multiply, and 4 to divide. I am facing the issue of making it so that you must enter a number, however instead of it being any number, it must be 1, 2, 3, or 4, and if it is not any of those numbers, you must re-enter the number. Here is a little snippet of my code:

Code:

printf("Please enter 1 to add, 2 to subtract, 3 to multiply, or 4 to multiply: ");
while (scanf("%d", &input) != 1) {
while (getchar() != '
');
printf("Invalid option. Please try again: ");

[Code] ....

how I can make the loop affect specific numbers.