kb pascal

var
        i,n,res:longint;
        s:string;
        d:array [1..200] of longint;
procedure nhap;
begin
        readln(n);
        readln(s);
end;
procedure giai;
var i:longint;
begin
        if n>26 then
        begin
            write(-1);
            exit;
       end;
        for i:=1 to n do
                inc(d[ord(s[i])]);
        for i:=ord('a') to ord('z') do
                if d[i]>1 then
                        res:=res+(d[i]-1);

        writeln(res);
end;
begin
        assign(input,'kb.inp');reset(input);
        assign(output,'kb.out');rewrite(output);
        nhap;
        giai;

end.