Firebase – Reactjs – Change the value of state based on a input field when the state has a default value?

I am working with a Firebase project and I have a project in my database which has a pledge1 field with a default value of 25.

I have two input fields where one is a button and the other is for text. I want the button to have a default value of "Pledge $25" and the input field to represent an additional amount of money to pledge. And I want the button’s value to update based on how much additional value is in the other input field.

So if a user decides to add additional $10, the button’s value should update to "Pledge $35" and if the user deletes the "0" from the "10", the button will update to say "Pledge $26"

This is what I have so far:

useEffect(() => {
    onSnapshot(q, (snapshot) => {
        let project = [];
        snapshot.docs.forEach((doc) => {
            project.push({ ...doc.data(), id: doc.id });
        })
        setProject(project)
        let amount = 0;
        project.map((project) => {
            amount = parseInt(project.pledge1);
        })
        setPledgeAmount(amount);
        setOriginalPledgeAmount(amount);
    })
}, [])

With this useEffect function I wait until the data is grabbed from firebase, and I set the state to the pledge amount. I then display that state in the input fields:

<div className="pledge-input">
    <input
        type="input"
        className="pledge-input-amount"
        placeholder="10"
        onChange={(e) => {
            let number = e.currentTarget.value;
            if (number >= 1) {
                setPledgeAmount(pledgeAmount + parseInt(number));
            }
            if (number === "") {
                setPledgeAmount(originalPledgeAmount);
            }
        }}
    />
    <input
        type="button"
        className="pledge-btn"
        value={`Pledge $${pledgeAmount}`}
    />
</div>

This only works if you add a single digit number and delete it, the problem is that if you write double digit numbers or higher,like 10, the value goes from 25 -> 26 -> 36 because it adds each digit that is added to the input field. And each time you delete a digit it adds the remaining digits to the state.

How can I make it only register the numbers added as entire numbers so when a user types in 50, the value of the state goes from 25 -> 75 and not 25 -> 30 -> 80

And how do I make it so when a digit is deleted it subtracts from the state and does not add to it. So if the additional value is 50 (25 50 = 75) and the zero gets deleted, it goes from 75 -> 30 (25 + 5) and not from 75 -> 80 (75 + 5)?