function extiende(){  
	var alto=(document.getElementById('pie').offsetHeight+document.getElementById('sup').offsetHeight+document.getElementById('msup').offsetHeight+document.getElementById('cont').offsetHeight);
	var total=(document.body.clientHeight);
	var actual=document.getElementById('extend').offsetHeight;
	var padbody=10;
	document.getElementById('extend').style.height=actual+(total-alto-padbody*2);
}