escaping infinite loop and return previous step automatically?

in here, if I input 0 for a, infinite loops occurs, what I want is escaping this infinite loop and returning previous step which asking me again input a,b,c? I wonder that can matlab execute this automatically? For example after the entering 0 matlab should ask me again a,b,c? automatically.

a=input(' a =? ')
b=input(' b =?')
c=input(' c =?')
while a==0
    disp(' equation is first degree, try to input another number except 0')
end

D=b^2-4*a*c
x1=( -b + sqrt(D) )/(2*a)
x2=( -b - sqrt(D) )/(2*a)
if D < 0
   disp(...........')
   x1
   x2
else if D == 0
      disp(...........')
      x1,x2
      else 
         disp(.............')
         x1
         x2
end

end

Answer 1

[Walter Roberson]

a = 0;

while a==0

    a=input(' a =? ')
    b=input(' b =?')
    c=input(' c =?')
    if a == 0
      disp(' equation is first degree, try to input another number except 0')
    end
  end

[sermet]

thank you so much