I think my program is correct, and I've checked it with the datas mentioned in the discuss.Would you please tell me what's wrong with my program? Thanks a lot!
First I find out the spaces which are reachable, then I count the sum of walls.
const poi:array[1..4,1..2]of shortint=((1,0),(0,1),(-1,0),(0,-1));
var i,j,k,t,n,m:longint;
    map:array[1..33]of string;
    form:array[1..33,1..33]of shortint;
    wall:array[0..34,0..34]of shortint;
    w:char;

procedure step(a,b:integer);
var i2,c,d:integer;
begin
     if form[a,b]=0 then begin
        form[a,b]:=1;
        for i2:=1 to 4 do begin
            c:=a+poi[i2,1];
            d:=b+poi[i2,2];
            if (c>0)and(c<=n)and(d>0)and(d<=n) then begin
               if map[c,d]='.' then begin
                  step(c,d);
               end;
            end;
        end;
     end;
end;

begin
    readln(n);
    fillchar(map,sizeof(map),0);
    for i:=1 to n do begin
        map[i]:='';
        for j:=1 to n do begin
            read(w);
            while (w<>'.')and(w<>'#') do read(w);
            map[i]:=map[i]+w;
        end;
        readln;
    end;
    fillchar(form,sizeof(form),0);
    step(1,1);
    if form[n,n]=0 then step(n,n);
    fillchar(wall,sizeof(wall),0);
    for i:=1 to n do begin
        for j:=1 to n do if form[i,j]=1 then begin
            for k:=1 to 4 do begin
                if form[i+poi[k,1],j+poi[k,2]]=0 then begin
                   inc(wall[i+poi[k,1],j+poi[k,2]]);
                end;
            end;
        end;
    end;
    m:=0;
    for i:=0 to n+1 do for j:=0 to n+1 do m:=m+wall[i,j];
    m:=m-4;
    writeln(m*9);
end.

Edited by author 09.10.2004 03:38

Thx a lot! When I changed form to 0..34, it got AC!