# Keep ticks but change interval for labels

I have a basic 2D plot

Plot[x^2, {x, -20, 20}, PlotRange -> {0, 30},
Ticks -> {Range[-20, 20, 1], Range[0, 30, 1]}, ImageSize -> 1000]


I have added the tick ranges so that there will be ticks at every integer. But this makes each tick get its own label, which is too many labels. How can I keep the ticks, but change the label spacing to be, say, every five ticks?

I've searched for a simple explanation for how to do this, but have not been successful.

f1 = If[Mod[#, 5] == 0, #, {#, ""}] &;