Why does it takes to click to apply my function?

I'm trying to write a code that when you click the button it shows and when pressed again it hides images. I made that images display to none and I want it to change to block when pressed the button (so it shows the images).

It works, but only when you click twice. What should I do to activate it on one click?

HTML :

    <button id="categoryBtn" onclick="insta()">instagram users</button>

javascript :

function insta() {
var a = document.getElementById("instaBalas");

if (a.style.display == "none") {
    a.style.display = "block";
    }
else {
    a.style.display = "none";
    }
}