Конфликт между формой и аккордеоном (тег input)

<section class="ac-container">
    <div>
        <input id="ac-1" name="accordion-1" type="checkbox" />
        <label for="ac-1">О нас</label>
        <article class="ac-small">
 <form id="ply" method="post" action="http://"  accept-charset="UTF-8" class="form-container"> 
<div class="form-title">
Введите значение.</div>
<input type="text" class="form-field" maxlength="9" size="9" name="amount" value="50">
<input type="submit" form="ply" class="submit-button" value="Отправить"> 
</form> 
        </article>
    </div>
    <div>
        <input id="ac-2" name="accordion-1" type="checkbox" checked />
        <label for="ac-2">Как мы работаем</label>
        <article class="ac-medium">
            <p>Текст статьи... </p>
        </article>
    </div>
    <div><!--...--></div>
</section>
 
<style>
.ac-container{
    width: 400px;
    margin: 10px auto 30px auto;
}
.ac-container label{
    font-family: 'BebasNeueRegular', 'Arial Narrow', Arial, sans-serif;
    padding: 5px 20px;
    position: relative;
    z-index: 20;
    display: block;
    height: 30px;
    cursor: pointer;
    color: #777;
    text-shadow: 1px 1px 1px rgba(255,255,255,0.8);
    line-height: 33px;
    font-size: 19px;
    background: linear-gradient(top, #ffffff 1%,#eaeaea 100%);
    box-shadow:
        0px 0px 0px 1px rgba(155,155,155,0.3),
        1px 0px 0px 0px rgba(255,255,255,0.9) inset,
        0px 2px 2px rgba(0,0,0,0.1);
}
.ac-container label:hover{
    background: #fff;
}
 
.ac-container label:hover:after,
.ac-container input:checked + label:hover:after{
    content: '';
    position: absolute;
    width: 24px;
    height: 24px;
    right: 13px;
    top: 7px;
    background: transparent url(../images/arrow_down.png) no-repeat center center;
}
.ac-container input:checked + label:hover:after{
    background-image: url(../images/arrow_up.png);
}
.ac-container input{
    display: none;
}
.ac-container article{
    background: rgba(255, 255, 255, 0.5);
    margin-top: -1px;
    overflow: hidden;
    height: 0px;
    position: relative;
    z-index: 10;
    transition:
        height 0.3s ease-in-out,
        box-shadow 0.6s linear;
}
.ac-container input:checked ~ article{
    transition:
        height 0.5s ease-in-out,
        box-shadow 0.1s linear;
    box-shadow: 0px 0px 0px 1px rgba(155,155,155,0.3);
}
.ac-container article p{
    font-style: italic;
    color: #777;
    line-height: 23px;
    font-size: 14px;
    padding: 20px;
    text-shadow: 1px 1px 1px rgba(255,255,255,0.8);
}
.ac-container input:checked ~ article.ac-small{
    height: 140px;
}
.ac-container input:checked ~ article.ac-medium{
    height: 180px;
}
.ac-container input:checked ~ article.ac-large{
    height: 230px;
}
</style>