#include <stddef.h>

void rot_x(char *s, int x)
{
    if (s == NULL)
    {
        return;
    }

    if (x < 0)
    {
        x = 26 + x;
    }

    for (size_t i = 0; s[i]; i++)
    {
        if (s[i] >= 'a' && s[i] <= 'z')
        {
            s[i] = ((s[i] - 'a') + x) % 26 + 'a';
        }
        else if (s[i] >= 'A' && s[i] <= 'Z')
        {
            s[i] = ((s[i] - 'A') + x) % 26 + 'A';
        }
    }
}