style.css

*{
    padding: 0;
    margin: 0;
    box-sizing: border-box;
}

body{
    background-image: url("back.jpg");
    background-size:cover;
    background-position: center;
}
.Menu{
    background: rgba(15, 134, 88, 0.63);
    text-align: center;
}
.Menu ul{
    display: inline-flex;
    list-style: none;
    color: rgb(245, 246, 255);
}
.Menu li{
    width: 120px;
    margin: 15px;
    padding: 20px;
    font-size: 25px;
    float:inline-start;
}

.Menu li a{
    text-decoration: none;
    color: rgb(245, 246, 255);
}

.maindiv{
    width: 80%;
    height: 700px;
    position: absolute;
    left: 50%;
    top: 50%;
    background-image: url("slide1.jpg");
    background-size: 100% 100%;
    transform: translate(-50%,-50%);
    box-shadow: 3px 4px 8px black;
    animation: slider 25s infinite linear;


}

@keyframes slider{
    0%{ background-image: url(slide1.jpg);}
    20%{background-image: url(back1.jpg);}
        60%{background-image: url(slide2.jpg);}
        80%{background-image: url(slide3.jpg);}
    
}