Var
   s: string;
   c,i,n,t: longint;

Begin
     readln(input, c);
     readln(input, s);
     n:= 0; t:= 0;
     for i:= 1 to c do
         begin
         if (s[i] = 'n') then Inc(n);
         if (s[i] = 'b') then Inc(t, n);
         end;
     writeln(output, t);
End.