Temperature Converter

#include <iostream>
#include <string>

using namespace std;
double fiveNinths = 0.5555555555555556;

double FtoC(double f)
{
    return (f - 32) * fiveNinths;
}

double FtoK(double f)
{
    return (f - 32) * (fiveNinths)+273.15;

}

double CtoF(double c)
{
    return c * 1.8 + 32;
}

double CtoK(double c)
{
    return c + 273.15;
}

double KtoF(double k)
{
    return (k - 273.15) * 1.8 + 32;
}

double KtoC(double k)
{
    return k - 273.15;
}

int main()
{
    double temp;
    short tempSelect;
    string f = "F", c = "C", k = "K";

    system("cls");
    cout << "Temperature  Converter" << endl;
    cout << "1. Fahrenheit into Celsius/Kelvin" << endl;
    cout << "2. Celsius into Fahrenheit /Kelvin" << endl;
    cout << "3. Kelvin into Fahrenheit/Celsius" << endl;
    cout << "4. Exit." << endl;
    cin >> tempSelect;

    switch (tempSelect)
    {
    case 1:
        system("cls");
        cout << "Fahrenheit into Celsius/Kelvin Converter" << endl;
        cin >> temp;
        system("cls");
        cout << "Fahrenheit into Celsius/Kelvin Converter" << endl;
        cout << temp << "F = " << FtoC(temp) << c << endl;
        cout << temp << "F = " << FtoK(temp) << k << endl;
        system("pause");
        main();
        break;
    case 2:
        system("cls");
        cout << "Celsius into Fahrenheit /Kelvin Converter" << endl;
        cin >> temp;
        system("cls");
        cout << "Celsius into Fahrenheit /Kelvin Converter" << endl;
        cout << temp << "C = " << CtoF(temp) << f << endl;
        cout << temp << "C = " << CtoK(temp) << k << endl;
        system("pause");
        main();
        break;
    case 3:
        system("cls");
        cout << "Kelvin into Fahrenheit/Celsius Converter" << endl;
        cin >> temp;
        system("cls");
        cout << "Kelvin into Fahrenheit/Celsius Converter" << endl;
        cout << temp << "K = " << KtoF(temp) << f << endl;
        cout << temp << "K = " << KtoC(temp) << c << endl;
        system("pause");
        main();
        break;
    case 4:
        system("cls");
        cout << "Goodbye." << endl;
        return 0;
        break;
    default:
        main();
    }
}