How to add a number in setInterval()?

I would like to increase a number by 1 every 1 second by using setInterval().
However, it would change the number to a string. How do I add a number instead of a string?

    function addNum (){setInterval(() => {number.innerHTML += 1;
        }, 1000);
    }

    document.getElementById("Btn").addEventListener("click",addNum);

<span id = "number">0</span>
<button id = "Btn">+1/s</button>