.cont_img{
    width: 870px;
    margin: 90px auto 0;
    padding-bottom: 108px;
}
.img_div img:nth-child(1){
    width: 870px;
    display: block;

}
.img_div {
    width: 870px;
    position: relative;
}
.mask_img{
    display: none;
    position: absolute;
    width: 100%;
    height: 100%;
    background: rgba(15, 84, 136, .6);
}
.title_img{
   z-index: 2;
    position: absolute;
    left: 56px;
    top: 101px;
}
.hide{
    display: none;
}
.checked_img{
    z-index: 2;
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%,-50%);
}
.img_ul {
    width: 870px;
}
.img_ul li{
    float: left;
    position: relative;
    width: 425px;
    margin-top: 20px;
}
.img_ul li:nth-child(even){
    float: right;
}
.img_ul li img:nth-child(1){
    width: 425px;
}
.title2_img{
    z-index: 2;
    position: absolute;
    left: 32px;
    top: 35px;
}