Error in findViewById statement on Android

I'm new to Android and I'm having a hard time programming. This part of the code always has the error in R of findViewById . When putting the mouse on the error appears the information Cannot Revolve simble "R" , and just gives me options to create a variable, class with the name and etc.

@SuppressLint("WrongViewCast")
@Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_cadastro);

        nome = (EditText) findViewById(R.id.edtNome);
        RE = (EditText) findViewById(R.id.edtRE);
        senha = (EditText) findViewById(R.id.edtSenha);

        button4 = (Button) findViewById(R.id.button4);


        button4.setOnClickListener(this);

    }