So I'm on week 2 of the harvard cs50 online course (2021x version). We're supposed to write a program that encrypts a text, moving every letters ASCII code by a certain amount, decided by the user through the command line. [Here is the full problem.](https://cs50.harvard.edu/x/2020/psets/2/caesar/) I'm nearly done, but already when I try to run the program, (it compiles just fine), it tells me there is a **segmentation error**. I don't really understand the problem. I've read that the problem has to do with accessing a certain part of memory that isn't accessible?? How do I solve this problem? Thank you in advance!
Here is my code..

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

int main(int argc, string argv[])
{
    int k, i;
    if (argc == 2 && isdigit(argv[1]) && argv[1] > 0)
    {
        k = (int) argv[1];
        string plaintext = get_string("plaintext: ");
        printf("cyphertext: ");
        for (i = 0; i < strlen(plaintext); i++)
        {
            if (islower(plaintext[i]) || isupper(plaintext[i]))
            {
                printf("%c", (((plaintext[i] + k) - 97) % 26) + 97);
                return 0;
            }

        }
    }
    else
    {
        printf("Usage: ./caesar key");
        return 1;
    }
}

```