let last = document.getElementsByClassName("previous");
let next = document.getElementsByClassName("next");
let number = 0; 

function photoChange() {
    let transform = number * -220;
    
    document.getElementsByClassName("gallery")[0].style.transform =
    "translateX(" + transform + "px)";
}

function add() {
    if (number < 4) {
        number++;  
        if (number == 1) {
            last[0].style.opacity = "1";
        } else if (number == 4) {
            next[0].style.opacity = "0.3";
        }
        
        photoChange(number);
    } 
    
}
function less() {
    if (number > 0) {
        number--;
        if (number == 0) {
            last[0].style.opacity = "0.3";
        } else if (number == 3) {
            next[0].style.opacity = "1";
        }
        
        photoChange(number);
    }
    
}

next[0].addEventListener('click', add);
last[0].addEventListener('click', less);