* {
    margin: 0;
    padding: 0;
}

.main {
    position: absolute;
    width: 400px;
    height: 200px;
    background-color: #cccccc;
    /*margin: auto;*/
    top: 50%;
    left: 50%;
    margin-top: -100px;
    margin-left: -200px;
}

.left-top {
    width: 50px;
    height: 50px;
    border-radius: 0 0 50px 0  ;
    background-color: #ffff00;
    position: absolute;
}

.right-bottom {
    width: 50px;
    height: 50px;
    border-radius: 50px 0 0 0;
    background-color: #ffff00;
    position: absolute;
    bottom: 0;
    right: 0;
}

.left-bottom {
    width: 50px;
    height: 50px;
    border-radius: 0 50px 0 0;
    background-color: #ffff00;
    position: absolute;
    bottom: 0;3
    left: 0;
}

.right-top {
    width: 50px;
    height: 50px;
    background-color: #ffff00;
    border-radius: 0 0 0 50px;
    position: absolute;
    right: 0;
}