if(task.isSuccessful()){

                String uid = mAuth.getCurrentUser().getUid();
                DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();

                DatabaseReference  uidRef = rootRef.child("mentor Users").child(uid);



                ValueEventListener valueEventListener = new ValueEventListener() {
                    @Override
                    public void onDataChange(com.google.firebase.database.DataSnapshot dataSnapshot) {
                        if (dataSnapshot.exists()) {
                            //start students activity
                            startActivity(new Intent(MainActivity.this, StudentTariningFargment.class));

                        } else {
                            //start teachers activity
                            startActivity(new Intent(MainActivity.this, com.google.android.gms.common.oob.parentFragment.class));
                        }
                    }
                    //
                    @Override
                    public void onCancelled(DatabaseError databaseError)
                    {
                    }
                };
                uidRef.addListenerForSingleValueEvent(valueEventListener);
            }

            else{

                Toast.makeText(MainActivity.this,"login erroe", Toast.LENGTH_SHORT).show();
            }

        }