invalid input

[sworc66]

I need my program to output a message "invalid input" when the input is not correct by the user.

Currently my programs use is to allow the user to enter a number and the program outputs that number in words.

This is whats meant to happen:

Input: 30
Output: thirty
Input: 30 eight
Output: invalid input

This is what happens:

Input: 30
Output: thirty
Input 30 eight
Output: thirty

Can anyone help me with how to get the invalid input thing to work. Here is a snippet of my code:

int res;
      int num;
      res = sscanf( cmd, "%d", &num);
      switch(num){
      
      /*0 - 20*/
      
         case 0: printf("zero\n");
            break;
         case 1: printf("one\n");
            break;
         case 2: printf("two\n");
            break;
         case 3: printf("three\n");
            break;
         case 4: printf("four\n");
            break;