Code:
#include <iostream>
#include <windows.h>
using namespace std;
int main()
{
    int one, value;
    cout << "Select:\n1 - Dec >> Hex\n2 - Hex >> Dec\n\n";
    cin >> one;
    system("cls");
    if (one == 1)
    {
        cout << "Enter your decimal value: ";
        cin >> value;
        cout << "\n\nThe hexadecimal value of " << value << " is " << hex << value << "\n\n";
    }
    else if (one == 2)
    {
        cout << "Enter your hexadecimal value: ";
        cin >> hex >> value;
        cout << "\n\nThe decimal value of " << hex << value << " is " << dec << value << "\n\n";
    }
    else
    {
        cout << "Invalid selection\n\n";
    }
    system("pause");
    system("cls");
    return main();
}