#define _POSIX_C_SOURCE 200809L

#include <stdio.h>

int merge_files(const char *file_1, const char *file_2)
{
    FILE *a = fopen(file_1, "a");
    if (a == NULL)
    {
        return -1;
    }
    FILE *r = fopen(file_2, "r");
    if (r == NULL)
    {
        return -1;
    }

    int c;
    while ((c = fgetc(r)) != EOF)
    {
        if (fputc(c, a) == EOF)
        {
            return -1;
        }
    }

    fclose(a);
    fclose(r);

    return 0;
}