Every day at noon, a scheduled steamboat departs from Moscow to Astrakhan and from Astrakhan to Moscow. A steamboat departing from Moscow takes exactly four days to reach Astrakhan, then stays for two days, and at noon, two days after its arrival in Astrakhan, it departs for Moscow. A steamboat departing from Astrakhan takes exactly five days to reach Moscow and, after a two-day rest in Moscow, departs for Astrakhan. How many steamboats should operate on the Moscow - Astrakhan - Moscow route under the described conditions of movement?
From the condition, it is clear that the steamboat, which left from Moscow, will leave from it again exactly $4+5+2+2=13$ days later.

## Oтвет

13 steamboats.

## Problem