Enable Button If Something Change?

I have a simple html

HTML Code:
<input type=checkbox name="email" />
<input type=checkbox name="social_network" />
<button onclick="save_options();this.disabled=true;"onload="this.disabled=true;">Save</button>

I need the button enabled if I check one of the check boxes and disabled when I click itself. I can do this in Jquery pretty easily, but in javascript. So I try some code like Below and it doesn't work.

Code:
document.getElementsByTagName("input").onChange(function(){
document.getElementsByTagName("button").enabled = true;
});