Need some help with Calculator program

Here a demo of the program. I trying to figure out how to make it take more then 2 number without added more variable. I know you have to use some kind of loop just having a little bit of trouble figuring it out.... THANKS.....

Code:

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#include <iostream>
#include <cmath>
#include <stdio.h>
#include <windows.h>


using namespace std;

void ErrorMessage();



int main() {

        float FirstNumber;
        float SecNumber;
        char Sign;
        float Equal;

        cout << endl;
        cout << "\t\t\t\tEnter Math Problem.\n";

        while ( true ) {


       
                cout << ">";
                cin >> FirstNumber >> Sign >> SecNumber;


                switch ( Sign ) {

                        case '+':

                                Equal = ( FirstNumber + SecNumber );
                                       
                                cout << endl;

                                cout << FirstNumber << " " << Sign << " " << SecNumber << " = " << Equal << "\n";

                                cout << endl;

                                break;

                        case '-':

                                Equal = ( FirstNumber - SecNumber );

                                cout << endl;

                                cout << FirstNumber << " " << Sign << " " << SecNumber << " = " << Equal << "\n";

                                cout << endl;

                                break;

                        case '*':

                                Equal = ( FirstNumber * SecNumber );

                                cout << endl;

                                cout << FirstNumber << " " << Sign << " " << SecNumber << " = " << Equal << "\n";

                                cout << endl;

                                break;

                        case '/':
                                       
                                if  ( ( FirstNumber == 0 ) || ( SecNumber == 0 ) ) {
                                               
                                        ErrorMessage();

                                        continue;
                                       
                                } else {

                                                Equal = ( FirstNumber / SecNumber );
               
                                                cout << endl;

                                                cout << FirstNumber << " " << Sign << " " << SecNumber << " = " << Equal << "\n";

                                                cout << endl;

                                                break;
                                }

                        default:

                                ErrorMessage();

                                cout << "You can't do that!\n\n";

                                continue;

                }

        }

       
       
        system("pause");

        return(0);


}



void ErrorMessage() {

        cout << endl;

        cout << "ERROR!!! \n";

        cout << endl;

}

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Why is FirstNumber equal to zero on divide an error? It's a valid operation.

Well if you were doing subtraction for instance,

a - b - c

you would first subtract b from a and then finally c from the difference. So your loop control would be how many terms there is going to be. In this case, there are two, as you would read a and b and then c.

It may be helpful to use a sentinel value like zero, to signal that the last term is complete.

If you have 1 + 2 * 3 that won't work. It is more complicated. If not, if you only + and - or * and / then you will have:
firstNumber operator OtherNumber operator OtherNumber .... OtherNumber = Result

So you read the first number, save it in TempResult and then in a loop you do:
1) read operator
2) read OtherNumber
3) TempResult = TempResult operator OtherNumber

And you are done.