reprompt user is char and int is combined

Question

Hey guys,
My code works fine, the only thing is, it allows you to input a letter with the "key". For an example, when I go to run it, I have to write: ./caesar , then the key: ./caesar 1 or ./caesar 2, any number, it doesn't matter. The only issue is it will not repromt you if you put a letter after ./caesar. example: ./caesar 2x. If anyone has any idea how to fix this, id really appreciate it!
THANK YOU

#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>

int main(int argc, string argv[])
{
if (argc != 2)
{
printf("Usage: ./caesar key\n");
return 1;
}

int k = atoi(argv[1]);
do
{
string p = get_string("plaintext: ");
printf("ciphertext: ");

for (int i = 0, n = strlen(p); i < n; i++)
{
if (islower(p[i]))
{
printf("%c", (((p[i] + k) - 97) % 26) + 97);
}
else if (isupper(p[i]))
{
printf("%c", (((p[i] + k) - 65) % 26) + 65);
}
else
{
printf("%c", p[i]);
}

}
printf("\n");
return 1;
}
while (argc == 2);

}

Answer 1

Iterate through argv[1] before you call atoi and test if every character is a number. ( or whatever other valid character )
Display error and return if it contains any invalid character.

Answer 2

how are you using string in C like in C++???

Answer 3

✳AsterisK✳ im just pulling it from the cs50 library.