1. Use $1,2,3,4,5$ to form a five-digit number, such that the difference between any two adjacent digits is at least 2. Then the number of such five-digit numbers is $\qquad$ .
2.1.14.
Consider $\square$ classified by the middle number $a$:
(1) If $a=1$, then 2 and 3 are on opposite sides of $a$, and 4 and 5 are on opposite sides of $a$, resulting in 4 permutations:
$$
24135,24153,35142,53142 \text {; }
$$
(2) If $a=2$, then 4 and 5 are on opposite sides of $a$, and 1 and 3 are not adjacent to $a$, resulting in 2 permutations:
14253,35241 ;
(3) If $a=3$, then 1 and 2 are on opposite sides of $a$, and 4 and 5 are on opposite sides of $a$, with 2 and 4 not adjacent to $a$, resulting in 2 permutations:
25314,41352 ;
(4) By symmetry, $a=5$ is the same as (1), with 4 permutations; $a=4$ is the same as (2), with 2 permutations.
Therefore, there are a total of 14 such five-digit numbers.