private boolean checkPermission() {
        return ContextCompat.checkSelfPermission(MainActivity.this,
                android.Manifest.permission.WRITE_EXTERNAL_STORAGE) == PackageManager.
                PERMISSION_GRANTED;
    }

    private void requestPermission() {
        if (ActivityCompat.shouldShowRequestPermissionRationale(MainActivity.this,
                android.Manifest.permission.WRITE_EXTERNAL_STORAGE)) {
            Toast.makeText(MainActivity.this, "Write External Storage permission allows us to " +
                    "do store rescue package. Please allow this permission in App Settings.",
                    Toast.LENGTH_LONG).show();
            showOneTimeAd();
        } else {
            Toast.makeText(MainActivity.this, "Write External Storage permission allows us to " +
                            "do store rescue package. Please allow this permission",
                    Toast.LENGTH_LONG).show();
            ActivityCompat.requestPermissions(MainActivity.this,
                    new String[]{android.Manifest.permission.WRITE_EXTERNAL_STORAGE},
                    PERMISSION_REQUEST_CODE);
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String permissions[], @NonNull int[] grantResults) {
        switch (requestCode) {
            case PERMISSION_REQUEST_CODE:
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    Log.i(TAG, "Permission Granted, Now you can copy rescue package .");
                } else {
                    Log.i(TAG, "Permission Denied, You cannot copy rescue package .");
                    Toast.makeText(ctx,"Failed, Rescue package cannot be copied, " +
                            "You should grant required permission",Toast.LENGTH_LONG).show();
                }
                break;
        }
    }