Adding CartoDB basemap in OpenLayers 3?

I’m trying to add a CartoDB basemap to my OpenLayers map but I can’t get it to work. Here is the code I use to declare the basemap variable:

const carto = new ol.layer.Tile({ 
    source: new ol.source.XYZ({ 
        url:'http://{s}.basemaps.cartocdn.com/light_all/{z}/{x}/{y}.png',
    })
});

Any idea where the problem comes from?

const carto = new ol.layer.Tile({ 
    source: new ol.source.XYZ({ 
        url:'http://{1-4}.basemaps.cartocdn.com/light_all/{z}/{x}/{y}.png',
    })
});