Javascript, error when changing Class value by +=

Question

my second post on StackOverflow!! :)

I am working on a project, were I've created a class and I want to transfer value by using a input / submit field in HTML. Now I get below error:

• It does not add 10 to 100, so it should be 110.. instead, it is adding it on the back, so it should be 110, but the value is then 10010. When submitting again, it becomes 1001010;

Anyone who is able te help me? Any feedback is welcome. (Yes I am a beginner :):):))

class Person {
    constructor(name) {
        this.name = name;
        this.bank = 1000;
        this.inbank = 100;}}

function pocketToBank() {
    var qq = document.getElementById("pockettobank").value;
    person1.inbank += qq;
    person1.bank -= qq;
    updateStats();
};

<form name="myForm">
 <label for="pockettobank">Pocket to Bank</label>
 <input type="number" id="pockettobank" name="pockettobank" required>
 <button onclick="pocketToBank(); return false">Submit</button>
</form>