Greatest common divisor:
begin
   int x, y;
   read x;
   read y;
   if 0=x*x+y*y then write 0-1
   else begin
      if x<0 then x:=0-x else;
      if y>=0 then else y:=0-y;
      while x*y\=0 do 
         if x>y then x:=x-y 
         else y:=y-x;
      write x+y;
   end
end.
x=0 and y=gcd(x',y') or
y=0 and x=gcd(x',y').