printf("Dwste ISBN tou epi8umhtou bibliou pros daneismo:\n");
    gets(n);
    fp=fopen("books","r");
    while (( !feof(fp) ) && (found!=0) )
    {
        fread(&b,sizeof(b),1,fp);
        if (!feof(fp) && strcmp(n,b.isbn)== 0 )
        {
            found=0;
            printf("to biblio uparxei sth biblio8hkh:\n");
            printf("%s %s %d \n" ,b.isbn,b.titlos,b.antitupa);

            copies+=b.antitupa;
            printf("copies= %d\n",copies);

            fp2=fopen("daneismoi","a+");

            if (fp2==NULL)
            {
               printf("file not found!\n");
               break;
            }
            fseek(fp2, 0, SEEK_END); // seek to end of file
            int size = ftell(fp2);
            if (size==0)
            {
               printf("Empty file!\n");
               fclose(fp2);
               break;
            }
            fseek(fp2, 0, SEEK_SET);
            while ( !feof(fp2) )
            {
                fread(&d,sizeof(d),1,fp2);
                while (strcmp(n,d.isbn)== 0 )
                {     copies--;
                }
            }
            fclose(fp2);

            printf("diathesima antitupa= %d\n", copies);