regarding clearing the input buffer in c

Question

I have this code, a program to take an integer from a user that's negative, if not trap them in a loop until they enter a negative integer. I have this part down, however, how could i clear the input buffer?

I tried to use while ((getchar()) != ‘\n’); after the first scanf_s but it fails.

here is code

#include <stdio.h>

int main()

{
int num;

printf("Please enter a negative integer");
scanf_s("%d", &num);

while (num >= 0.0)
{
    printf("Please enter a negative integer!\n");
    scanf_s("%d", &num);
}

    printf("%d", num);


system("pause.exe");
return 0;
}