I got Crash (stack overflow) when my solution met it.

type ac=array[1..601]of longint;
var a,c,mi,lan,ch:ac;
b,d,e,f,g,h,i,j,k,l,m,n:longint;t:array[1..601]of char;
function times(a,c:ac;n:longint):ac;
var i,j,l:longint;
begin
 fillchar(ch,sizeof(ch),0);
 for i:=n downto 1 do begin
  l:=0;
  for j:=n downto 1 do
   if i+j-n>0 then begin
    ch[i+j-n]:=ch[i+j-n]+a[i]*c[j]+l;
    l:=ch[i+j-n] div 10;
    ch[i+j-n]:=ch[i+j-n] mod 10;
   end;
 end;
 times:=ch;
end;
function larger(a,c:ac;n:longint):boolean;
var i:longint;
begin
 for i:=1to n do
  if a[i]>c[i] then exit(true)
  else if a[i]<c[i] then exit(false);
 exit(false);
end;
procedure goal(g:longint);
var i,j:longint;
begin
 if g=602 then begin
  for i:=601-m+1 to 601 do
   write(c[i]);
  WRITELN;
  halt;
 end;
 fillchar(lan,sizeof(lan),0);
 if odd(n) then
  for j:=2to (g-(601-m))*2 do
   LAN[j]:=a[j+(601-n)-1]
 else
  for j:=1to (g-(601-m))*2 do
   LAN[j]:=a[j+(601-n)];
 for i:=1to 10 do begin
  if i<10 then begin
   c[g]:=i;
   fillchar(mi,sizeof(mi),0);
   for j:=1to g-(601-m) do
    mi[j+g-(601-m)]:=c[j+(601-m)];
   mi:=times(mi,mi,(g-(601-m))*2);
  end;
  if i=10 then begin
   c[g]:=9;
   goal(g+1);
  end
  else if larger(mi,lan,(g-(601-m))*2) then begin
   c[g]:=i-1;
   goal(g+1);
  end;
 end;
end;
begin
 n:=0;
 repeat
  inc(n);
  read(t[n]);
 until eoln;
 for i:=1to n do
  a[601-n+i]:=ord(T[I])-48;
 h:=0;
 for i:=601 downto 1 do begin
  a[i]:=a[i]*2+h;
  h:=a[i] div 10;
  a[i]:=a[i] mod 10;
 end;
 for i:=1to 601 do
  if a[i]<>0 then break;
 n:=602-i;
 m:=(n+1)div 2;
 goal(601-m+1);
end.

I think it's OK.
Does anybody know what's wrong with it?