function openNav() {
    document.getElementById("mySidenav").style.width = "250px";
    document.getElementById("main").style.marginLeft = "20px";
    document.getElementById("main").style.marginLeft = "20px";
    document.getElementById("overlay").style.width = "100%";
    document.getElementById("overlay").style.height = "100%";
    document.getElementById("overlay").style.backgroundColor = "rgba(0,0,0,0.4)";            
}

function closeNav() {
    document.getElementById("mySidenav").style.width = "0";
    document.getElementById("main").style.marginLeft= "0";
    document.getElementById("overlay").style.width = "0%";
    document.getElementById("overlay").style.height = "0%";
}