I would like my setInterval function to increase by a decimal number every 1 second. However, it will only increase by strings. How do I fix this?

let btn = document.getElementById("btn");
let demo = document.getElementById("demo");

function addUp(){
setInterval(()=>{
demo.innerHTML+=0.5
},1000);
}

btn.addEventListener("click", addUp);

<span id = "demo">0</span>

<button id = "btn">
click here
</button>