Android - No me deja abrir el apk que acabo de crear

package com.example.pablo.calculator3000;

import android.os.Bundle;

import android.support.design.widget.FloatingActionButton;

import android.support.design.widget.Snackbar;

import android.support.v7.app.AppCompatActivity;

import android.support.v7.widget.Toolbar;

import android.view.View;

import android.view.Menu;

import android.view.MenuItem;

import android.widget.Button;

import android.widget.EditText;

import android.widget.TextView;

public class Calculator3000 extends AppCompatActivity {

    EditText txtn1, txtn2;

    Button btnS, btnR, btnM, btnD;

    TextView txtR;

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_calculator3000);

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);

        setSupportActionBar(toolbar);

        txtn1 = (EditText) findViewById(R.id.txtn1);

        txtn2 = (EditText) findViewById(R.id.txtn2);

        btnS = (Button) findViewById(R.id.btnSuma);

        btnR = (Button) findViewById(R.id.btnResta);

        btnM = (Button) findViewById(R.id.btnMulti);

        btnD = (Button) findViewById(R.id.btnDiv);

        txtR = (EditText) findViewById(R.id.txtRes);

        btnS.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {

                int num1 = Integer.parseInt(txtn1.getText().toString());

                int num2 = Integer.parseInt(txtn2.getText().toString());

                int s = num1 + num2;

                txtR.setText("Resultado: " + s);

            }

        });

        btnR.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {

                int num1 = Integer.parseInt(txtn1.getText().toString());

                int num2 = Integer.parseInt(txtn2.getText().toString());

                int s = num1 - num2;

                txtR.setText("Resultado: " + s);

            }

        });

        btnM.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {

                int num1 = Integer.parseInt(txtn1.getText().toString());

                int num2 = Integer.parseInt(txtn2.getText().toString());

                int s = num1 * num2;

                txtR.setText("Resultado: " + s);

            }

        });

        btnD.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {

                int num1 = Integer.parseInt(txtn1.getText().toString());

                int num2 = Integer.parseInt(txtn2.getText().toString());

                int s = num1 / num2;

                txtR.setText("Resultado: " + s);

            }

        });

    }

}