Help with while loop

I can't do this while loop properly, someone help me please?

[Rik]

Why did you delete your previous question? It is the same as this one.

[Hdez]

Thank you for your help

Answer 1

[Walter Roberson]

dt = input('Enter the desired time step: ');
while dt >= h^2 / (4*alpha)
    fprintf('solution is unstable\nPlease enter a value smaller than %g\n', h^2/(4*alpha));
    dt = input('Enter the desired time step: ');
end