Untitled

<!DOCTYPE html>
<html>
<head>
	<meta charset="utf-8">
</head>
<body>

<form>
	<select id = "select1">
		<option>0</option>
		<option>1</option>
		<option>2</option>
		<option>3</option>
		<option>4</option>
		<option>5</option>
		<option>6</option>
		<option>7</option>
		<option>8</option>
		<option>9</option>
	</select>

	<select id = "select2">
		<option>0</option>
		<option>1</option>
		<option>2</option>
		<option>3</option>
		<option>4</option>
		<option>5</option>
		<option>6</option>
		<option>7</option>
		<option>8</option>
		<option>9</option>
	</select>

	<label id = "label_plus">+</label>
	<input id = "plus" type="checkbox" name="plus">
	<label id = "label_minus">-</label>
	<input id = "minus" type="checkbox" name="minus">
	<label id = "label_div">div</label>
	<input id = "div" type="checkbox" name="div">
	<label id = "label_mod">mod</label>
	<input id = "mod" type="checkbox" name="mod">

	<button id = "buton"> = </button>
</form>

<script type="text/javascript">
	window.onload = function(){
		var s1 = document.getElementById("select1");
		var s2 = document.getElementById("select2");
		var btn = document.getElementById("buton");
		var plus = document.getElementById("plus");
		var minus = document.getElementById("minus");
		var div = document.getElementById("div");
		var mod = document.getElementById("mod");
		btn.onclick = function() {
			if(plus.checked == true){
				var p = document.createElement("p");
				p.innerHTML = "1";
				document.body.appendChild(p);
			}
			if(minus.checked == true){
				console.log("-");
			}
			if(div.checked == true){

			}
			if(mod.checked == true){

			}
		}
	}
</script>

</body>
</html>