public static void formatInput(final String pf, final EditText et) {
            et.setText(pf);
            final TextWatcher tw = new TextWatcher() {
                String lastText = et.getText().toString();

                @Override
                public void onTextChanged(CharSequence s, int start, int before, int count) {
                    String input = s.toString();
                    if (TextUtils.isEmpty(input)) {
                        lastText = pf;
                        et.setText(pf);
                        return;
                    }
                    if (!input.startsWith(pf)) {
                        et.setText(lastText);
                        return;
                    }
                    if (input.startsWith(pf)) {

                    }
                    lastText = input;
                }

                @Override
                public void beforeTextChanged(CharSequence s, int start, int count, int after) {
                }

                @Override
                public void afterTextChanged(Editable s) {
                    et.setSelection(et.getText().toString().length());
                }
            };
            et.gette
            et.addTextChangedListener(tw);
        }