Reworked variable d from an int to a char...

... now it reads only the first char from the data file and if it is not a '1' || '0' the program will terminate.

ex1. if the data file reads '1', '2', '3'... the program will interpret only the first char of '1' and continue

ex2. if the data file reads ' ', '1', '2'... the program will interpret the null char and terminate

ex3. if the data file reads 'a', 'b', '1'... the program will interpret the 'a' char and terminate