i've got wa on test2. In fact my algo is very simple and a know that this program want another algo but can't understand why this algo don't work...
var
    n,x:int64;
    s,t:array [1..250000] of char;
    bool:boolean;
    i,j:longint;
begin
   readln (n);
   for i:=1 to n-1 do
      read (s[i]);
   readln (s[n]);
   for i:=1 to n-1 do
      read (t[i]);
   readln (t[n]);
   x:=-1;
   for i:=1 to n do
      if s[i]=t[1] then
      begin
         bool:=true;
     for j:=2 to n-i+1 do
        if s[j+i-1]<>t[j] then
        begin
           bool:=false;
           break;
        end;
     if bool then
        for j:=1 to i-1 do
           if s[j]<>t[n-i+j+1]then
           begin
          bool:=false;
          break;
           end;
     if bool then
     begin
        x:=n-i+1;
        break;
     end;
    end;
   writeln (x);
end.
 Regards Thefox