SOURCE

var inputOne = document.getElementsByTagName('input')[0],
    inputTwo = document.getElementsByTagName('input')[1],
    oButton = document.getElementsByTagName('button')[0],
    //oSpan = document.getElementsByTagName('span')[0],
    num = 0;
oButton.onclick = function(){
  for(var i = parseInt(inputOne.value);i<=parseInt(inputTwo.value);i++){
    num += i ;
  }
  alert(num);
  num = 0;
  //oSpan.innerHtml = num;
}

<input type="text" /> +
<input type="text" />
<button>求和</button> <!--=
<span></span>-->

input{
  width:50px;
}
span{
  float:left;
}