Why?
code pascal
program z1;
var s, s1:string;
    n, x:longint;
begin
  readln(n);
  readln(s);
  readln(s1);
  if s=s1 then write(0)
  else begin
  s:=s+s;
  case pos(s1, s) of
  1..250000:writeln(n-pos(s1, s)+1);
  else write(-1);
  end;
  end;
  readln;
  end.

Your algorithm is wrong, read more about KMP algorithm or string hashing

P.S. Test for you:
aaa...aaa
aaa...aab