#include <iostream>
#include <string>
#include <iomanip>

using namespace std;

int main() {
    double fah = 0;
    double cel = 0;
    string input = " ";
    string yn = " ";

    cout << "Welcome to the temp calculator" << endl;
    cout << "-------------------------------" << endl;

    do{
        cout << "What would you like to convert 'Fah-Cel' or 'Cel-Fah' ";
        cin >> input;


        while (toupper(input) == "FAH-CEL") {
            cout << "What is the Fah temp?: ";
            cin >> fah;
            cel = 5 * (fah - 32) / 9;
            cout << fixed << setprecision(2);

            cout << "The cel conversion is " << cel << " degrees" << endl;
            break;
        }
 
        while (toupper(input) == "CEL-FAH") {
            cout << "What is the Cel temp?: ";
            cin >> cel;

            fah = cel * (9 / 5) + 32;
            cout << fixed << setprecision(2);
            cout << "The fah conversion is " << fah << " degrees" << endl;
            break;
        }

        if (cel >= 28 or fah >= 85) {
            cout << "Boy it is gonna be hot" << endl;
        }


        if (cel <= 28 or fah <= 85) {
            cout << "Boy it is gonna be cold" << endl;
        }
        
        cout << "Would you like to do another calculation?: ";
        cin >> yn;


    } while (toupper (yn) == "YES");

        cout << "-----------------------------------------------" << endl;
        cout << "Thank you for using the program" << endl;


    return 0;
}

I am quite new to C++ so I am trying to do some basic terminal projects but I dont know why the touppers wont work here. If I remove them then the program runs fine, but if I try running it how it is VS gives me a build error. I am just trying to find a way so that the inputs arent case sensitive. Is there another way to have it to ensure users ease of use?