.slider {height:200px; width:100%; margin:auto; overflow:hidden;}

.slider ul {display:flex; padding:0; width:400%; animation:cambio 20s infinite alternate ease-in;}

.slider li {width:100%; list-style:none;}

.slider img {width:100%;}

@keyframes cambio {
    0% {margin-left: 0;}
    20% {margin-left: 0;}
    
    25% {margin-left: -100%;}
    45% {margin-left: -100%;}
    
    50% {margin-left: -200%;}
    70% {margin-left: -200%;}
    
    75% {margin-left: -300%;}
    100% {margin-left: -300%;}
}