Flex slider/container

index.html

<div class="slider">
   <div class="item item-1">
       <span class="item-title">Summer</span></div>
   <div class="item item-2">
       <span class="item-title">Is</span></div>
   <div class="item item-3">
       <span class="item-title">Very</span></div>
   <div class="item item-4">
       <span class="item-title">Cool</span></div>
</div>

style.css

.slider {
    display: flex;
    flex-direction: row;
    justify-content: center;
    align-items: center;
    width: 80vw;
    margin: auto;
}

.item {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 70vh;
    flex-basis: 25%;
    color: white;
    text-transform: uppercase;
    font-size: 2em;
    transition: all .5s ease;
    cursor: pointer;
    position: relative;
}
.item:after {
    content: '';
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    display: block;
    background-color: rgba(0, 0, 0, 0.5);
    transition: all .5s ease;
}
.item .item-title {
    z-index: 1000;
}
.item:hover {
    flex-basis: 100%;
}
.item:hover:after {
    background-color: rgba(0, 0, 0, 0.3);
}

.item-1 {
    background: url('https://source.unsplash.com/random/700x1001');
    background-position: center;
}
.item-2 {
    background: url('https://source.unsplash.com/random/700x1002');
    background-position: center;
}
.item-3 {
    background: url('https://source.unsplash.com/random/700x1003');
    background-position: center;
}
.item-4 {
    background:  url('https://source.unsplash.com/random/700x1004');
    background-position: center;
}

@media (max-width: 768px) {
    .slider {
        height: 100vh;
        width: 100vw;
        flex-flow: column;
    }
    .item {
        width: 100%;
    }
    .item:hover {
        flex-grow: 30%;
    }
}