How do I write this C++ sentinel-controlled while loop?

I need to write a sentinel-controlled while loop to see if the coefficients of a quadratic equation have real roots using a nested decision. A character q must be my sentinel. This is what I have so far, but it doesn't really work and I'm getting lost...

# include <iostream>

# include <iomanip>

# include <cmath>

using namespace std;

int main ()

{

//Declare Variables

double a, b, c, r1, r2, descriminant;

char q;

char SENTINEL = q;

cout << setiosflags (ios::fixed)

<< setiosflags (ios::showpoint)

<< setprecision (3);

cout << "Input the Value for a or q to Quit: " << endl;

cin >> a; // read a

while (a!= SENTINEL)

{

if (a == 0)

cout << "Zero Divide" << endl;

else if (a != 0)

cout << "Input the Value for b or q to Quit: " << endl;

cin >> b; // read b

cout << "Input the Value for c or q to Quit: " << endl;

cin >> c; // read c

if (descriminant < 0)

cout << "No Real Roots" << endl;

else if (descriminant >= 0)

cout << setw(6) << "The Real Roots Are: " << endl;

cin >> r1; //read r1

cin >> r2; //read r2

descriminant = pow(b,2) - (4 * a * c);

r1 = (-b + (pow(b,2) - (4 * a * c))) / (2 * a);

r2 = (-b - (pow(b,2) - (4 * a * c))) / (2 * a);

cout <<"Root One is: " << r1 << endl;

cout <<"Root Two is: " << r2 << endl;

}

system ("PAUSE");

return 0;

}