OffsetHeight - Get The Height Of A <div> Tag

I'm trying to get the height of a <div> tag so I can use it as a reference in a function to change image height, but I can't seem to get it.

function getdimensions()
{
RightContent = document.getElementById("Right").offsetHeight;
LeftContent = document.getElementById("Left").offsetHeight;
return LeftContent, RightContent;
}
window.onload = function() {
getdimensions();}

I have that, and the div tags have the corresponding ID's, but when I try one of these html buttons to test I just get 'undefined'.

<button onclick="alert(RightContent.offsetHeight);">Right Height</button><br>
<button onclick="alert(LeftContent.offsetHeight);">Left Height</button>

What's going wrong?