Adding inputted value onto existing value on different activity

I'm looking to create a running total on my MainActivity.java, this will be done through adding integers calculated in ActivityAdd.java then sending them across onClick save to a TextView in the MainActivity.java.

Any advice on how i would do this? it currently sends the values but doesn't consider adding the value to an existing value. it just removes the old with the new. thanks for your time, find my code below.

ActivityAdd.Java

 OnClickListener button02OnClickListener =
            new OnClickListener(){

                @Override
                public void onClick(View v) {

                    TextView et1 = (TextView)findViewById(R.id.textView4);
                    String theText = et1.getText().toString();

                    Intent intent = new Intent(getApplicationContext(),MainActivity.class);
                    intent.putExtra("calorie", theText);
                    startActivity(intent);

                }};

ActivityMain.Java

String calorie = getIntent().getStringExtra("calorie");
        TextView textView1 = (TextView)findViewById(R.id.TextView1);
        textView1.setText(calorie);