Simple Vigenere cipher written in C

vigenere.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <cs50.h>
#include <ctype.h>
 
int main(int argc, char *argv[])
{ 
    if (argc != 2)
    {
        printf("Error code 1 Input is the Wrong Number of Arguments\n");
        return 1;
    }   
         
    else     
    {
        string k = argv[1];
        
        for(int e = 0; e < strlen(k); e++)
        // second loop
        // ensures each character is a letter
        {
            if (isalpha(k[e]))
            {continue;}
            if (!isalpha(k[e]))
            printf("Error Code 2 Argument %s is not a letter\n", k);
            return 1;
            {break;}
        }
        //take string of plaintext and begin encoding
        string plain = GetString();
        int i = 0;
        for (int j = 0; j < strlen(plain); j++)
        {   
        if (isalpha(plain[j]) == 0)
        {
            //leave special chars alone
            printf("%c", plain[j]);
            i++;
        }
        else
        {
            char a = plain[j];
            int x = a;
            int l = ((j - i) % strlen(k));
            char m = k[l];
            int d = m;
                    
            if (x > 64 || x < 91)
            //lower case chars
            {
                if (d < 91)
                {
                    int e = d - 65;
                    int v = e + x;             
                    if (v > 90)
                    {
                        int o = v - 26;
                        printf("%c", o);
                    }
                    else
                    {
                        printf("%c", v);
                    }
                }
                else
                //upper case chars
                {
                    int e = d - 97;
                    int v = e + x;              
                    if (v > 122)
                    {
                        int o = v - 26;
                        printf("%c", o);
                    }
                    else
                    {
                        printf("%c", v);
                    }
                }
            }            
        }   
    }
    printf("\n");
    return 0;
    }
}

this was used in harvard's CS50, and requires one of their libraries (cs50.h) to compile correctly. As time permits I will rewrite it to remove the dependency, as it is really not required. The dependencies are the function GetString() and the struct "string".