Error:(31, 40) error: incompatible types: android.support.v4.app.Fragment cannot be converted to android.app.Fragment

I have a problem in this code and I don't know which one I should use android.support.v4.app.Fragment or android.app.Fragment; public class MainActivity extends AppCompatActivity {

private SharedPreferences pref;
@Override
protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); pref = getPreferences(0); initFragment();
}
private void initFragment(){ android.support.v4.app.Fragment fragment; if(pref.getBoolean(Constants.IS_LOGGED_IN,false)){ fragment = new ProfileFragment(); }else { fragment = new LoginFragment(); } FragmentTransaction ft = getFragmentManager().beginTransaction(); ft.replace(R.id.fragment_frame,fragment); ft.commit();
}

}