The cost of conventional medications for diabetes has risen by an average of 13.1% per year over the past four years. During this same period of time, the cost of a specialty drug for rheumatoid arthritis (RA) has increased 22.5% per year. If these increases continue into the foreseeable future, in how many years will an RA medicine now costing $20 per dose be at least five times as expensive as a diabetes inhaler that now costs $10 per use?