#include <iostream>
#include <string>
#include <sstream>
using namespace std;
int main()
{
    string hexadecimal;
    //convert hexadecimal to string characters
    int len, counter =0, variable=0, valid;
    char h;

    cout<<"This program converts hexadecimal numbers to decimal numbers."<<endl
    <<"Enter a hexadecimal number: ";

    cin>>hexadecimal;

    len = hexadecimal.length(); //find length of function
    for ( counter = 0; counter < len; counter++)
        {h =hexadecimal.at(counter); //retrieve individual character

        if (h>='0' && h<='9')
            {valid = h - '0';}

        else if (h>= 'A' && h<= 'F')
            {
                switch (h)
                {
                case('A'): valid = 10;
                    break;
                case('B'): valid = 11;
                    break;
                case('C'): valid = 12;
                    break;
                case('D'): valid = 13;
                    break;
                case('E'): valid = 14;
                    break;
                case('F'): valid = 15;
                    break;
                }
            } //convert letters to numbers

          else if (h>= 'a' && h<= 'f')
            {
                switch (h)
                {
                case('a'): valid = 10;
                    break;
                case('b'): valid = 11;
                    break;
                case('c'): valid = 12;
                    break;
                case('d'): valid = 13;
                    break;
                case('e'): valid = 14;
                    break;
                case('f'): valid = 15;
                    break;
                }
            } //convert letters to numbers
        else if (h = 'X')
            {cout<<"Goodbye";
            return 0;}
        else
            {cout<<"Invalid hexadecimal number";
                return 0;
            } //test character to be within range

        variable = variable * 16 + valid;
        }
        cout <<"The decimal number for "<<hexadecimal<<" is "<<variable;
        return 0;
    }