Edit in JSFiddle

$(document).ready(function() {
    $('#input_value').keyup(function() {
        try {
            $('#result').html(~~eval(this.value));
        } catch (e) {
            $('#result').html('invalid input')
        }
    }).keyup();
});

<div>
    <label for="input_value">input value to determine</label>
    <input type="text" name="input_value" id="input_value" value="'-2.3'" />
</div>
<hr />
<div>
    result:
    <span id="result" class="result"></span>
</div>