In C++

Write a program that takes in an input file name and an output file name as command line arguments, opens the file and reads in its contents, deciphers the encoded message contained in the input file, and outputs the plaintext message to an output file. Your program must take in two file names as command line arguments (one for the input file and one for the output file.) You may assume that only upper case letters will be used for the text. The first line of the encoded file will contain the rotational value, and the second line and following contains the encoded text.

Your program should run like this:

$ more encoded.txt

7 AXEEH PHKEW

$ g++ cs135prog.cpp

$ ./a.out encoded.txt decoded.txt

$ more decoded.txt

HELLO WORLD $

Help I don't understand how to do this! Thank you in advance!