var td = document.getElementsByTagName("td");
for(var i = 0; i < td.length; i++){
	if(td[i].className == 'letra'){
		var button = td[i];
		
		button.onmouseover = function(){
			this.style.background = '#DAF1FC';
		}		
		button.onmouseout = function(){
			this.style.background = 'none';
		}
	}
}