function $id(i) { return document.getElementById(i) };

addLoadEvent(function() {

    var select_single = $id('select_single'),
        other_single = $id('other_single'),
        input_single = $id('input_single'),
        select_month = $id('select_month'),
        other_month = $id('other_month'),
        input_month = $id('input_month');

    if (select_single) {
        select_single.onchange = function() {
            if (this.value == 'other') {
                this.style.display = 'none';
                this.name = 'amount_select';
                input_single.name = 'amount';
                other_single.style.display = '';
            }
        };
    }

    if (select_month) {
        select_month.onchange = function() {
            if (this.value == 'other') {
                this.style.display = 'none';
                this.name = 'amount_select';
                input_month.name = 'a3';
                other_month.style.display = '';
            }
        };
    }

});