sarnjak,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.necessarily{
background: #DCDCDC
}
</style>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<script>
$(function () {
$(':submit').prop({
'disabled': true
})
$('form').on('input', function () {
var sum = 0;
$('.necessarily').each(function () {
sum += +this.value || 0
})
$(':submit').prop({
'disabled': sum == 0
})
})
})
</script>
</head>
<body>
<form>
<input type="submit" value="input" ><br>
<button type="submit" > button </button> <br>
<input class="test" type="text" value="">
<input class="necessarily" type="number" value="">
<input class="necessarily" type="number" value="">
<input class="test1" type="text" value="">
</form>
</body>
</html>