Read this and answer the question. If the question is unanswerable, say "unanswerable".

BeiDou-1 is an experimental regional navigation system, which consists of four satellites (three working satellites and one backup satellite). The satellites themselves were based on the Chinese DFH-3 geostationary communications satellite and had a launch weight of 1,000 kilograms (2,200 pounds) each.

How many satellites in the BeiDou-1 system is used for back up?
one