minetest.register_on_joinplayer(function(player)
    player:set_hp(20)
    player:set_attribute("a:hp",100)
end)

minetest.register_on_player_hpchange(function(player, hp_change)

    if hp_change < 0 then
        local gethp = tonumber(player:get_attribute("a:hp"))
        local newhp = gethp + hp_change
        minetest.chat_send_all(newhp)
        player:set_attribute("a:hp",newhp)

        if newhp <= 0 then
            player:set_attribute("a:hp",100)
            return -20
        end
    end

    if hp_change ~= 20 then
        return 0
    else
        return 20
    end

end, true)