How to summarize firebase data in android?

Question

Java

Good afternoon! Essence of the question.
I have a field in Firebase, - balance.
What I need - after the person enters the amount - 100, the field changes the value to 100, then the person enters the amount 50, the field value becomes 150.
How can I write the processing logic on the client? Data summation.
I think this is oversimplistic, but I need your help!

mDatabaseUsers.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {



                String user_id = mAuth.getCurrentUser().getUid();
                
                String balance = dataSnapshot.child(user_id).child("Balance").getValue(String.class);

                mCountPayment.setText(balance + " ₽");

            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

        mPaymentButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                paymentIntent();



            }
        });
    }

    private void paymentIntent() {


        final String user_id = mAuth.getCurrentUser().getUid();

        final String count = mPaymentList.getText().toString().trim();

        if (!TextUtils.isEmpty(count)) {


            mDatabaseUsers.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {


                    mDatabaseUsers.child(user_id).child("Balance").setValue(count);


                }

                @Override
                public void onCancelled(DatabaseError databaseError) {

                }
            });


        }


    }

Answer 1

Two ways:
1. You read the previous value from firebase in advance, add the numbers on the client, and the result of the sum is already overwritten.
2. Look towards transactions