program noip5_2;
 var
    s1,s2:array[1..5000] of char;
    f:array[0..5000,0..5000] of integer;
    n:integer;

procedure init;
 var
    i:integer;
 begin
  readln(n);
  for i:=1 to n do
    begin
    read(s1[i]);
    s2[n-i+1]:=s1[i]
    end;
 end;

procedure main;
 var
    i,j:integer;
 begin
  for i:=1 to n do
   for j:=1 to n do
    begin
    f[i,j]:=f[i-1,j];
    if f[i,j-1]>f[i,j]
    then f[i,j]:=f[i,j-1];
    if s1[i]=s2[j]
    then if f[i-1,j-1]+1>f[i,j]
         then f[i,j]:=f[i-1,j-1]+1;
    end;
  writeln(n-f[n,n]);
 end;

begin
 assign(input,'palin.in');
 assign(output,'palin.out');
 init;
 main;
 close(input); close(output);
end.