int main(int argc, char **argv)

First get your arguments from the command line. Your program must accept one and only one argument not including the program name itself. That argument must be an integer. If anything else is entered in any way, terminate the program with a suitable error message telling the user how to use your program correctly. Remember, you may not use more than one return, even in main() and you may not use exit() or anything like that.

Don't use any libraries other than iostream and string