Error:(31, 40) error: incompatible types: android.support.v4.app.Fragment caot 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();
}

}