A subordinate clause in conditional sentences in the Subjunctive mood can be missing where its meaning is clear from the context.
We bought a short piano - a grand piano would occupy too much room.
Мы купили пианино, - рояль занял бы слишком много места.
You must be a stranger here otherwise you would know him.
Вы, наверно, не здешний, иначе вы бы знали его.