.slider {
    width: 999px;
    height: 100px;
    margin: auto;
    overflow:hidden;
}

.slider ul {
	display: flex;
    padding: 0;
    width: 300%;
    
    animation: cambio 20s infinite;
    animation-direction: alternate;
    
}
.slider li {
    width: 100%
    list-style: none;   
}

.slider imag {
	width: 100%;
}

@keyframes cambio {
    0% {margin-left: 0;} 
    15% {margin-left: 0;}
    
    20% {margin-left: -999;}
    30% {margin-left: -999;}
    
    40% {margin-left: -1998;}    
    60% {margin-left: -1998;}
        
    70% {margin-left: -2997;}    
    85% {margin-left: -2997;}
    
    90% {margin-left: -3996;}
    100% {margin-left: -3996;}
}