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).

Other examples of NPN formulas are:

3 ( result is 3)

x 2 0 (result is 0)

x + 2 3 + 2 2 (result is 20)

+ x 2 3 4 (result is 10)