Calculate PI using HTML

<div id="rpi"></div><hr>
<div id="pi"></div>
<script>
    //PI search
    function a (n){
        return (n/(n-1))*(n/(n+1))
    }
    c=2
    d=2
    function b(){
        d = d*a(c)
        c = c+2
        document.getElementById("pi").innerHTML = d +"<br>("+c/2+ " calculation)<br><strong>Calculating PI can take a while, you will need around 1,000,000 calculation.</strong>"
    }
    setInterval(b,1)
    document.getElementById("rpi").innerHTML = "Expected Output:<br>"+Math.PI
    // Calculation Inspired By 3Blue1Brown
</script>