Correctly compiles under the UNIX command line.

When run from the command line your program must read from standard input an NPN formula.

You can assume there is a space between every operator and number. Every number will be a 32 bit non-negative integer.

Your program should be able to correctly handle the binary operators: +, -, x.

IMPORTANT: be careful that your program recognises precisely the three characters above (plus sign, minus sign, and the lowercase letter x, respectively).

You may NOT use any Java standard library class or method other than for input/output.

Please provide your own code, I know there is the same question on this website.