题目描述 Given a positive integer $n$ ($3 \leq n \leq 90$), count all possible distinct binary strings of length $n$ such that there are no consecutive $1$'s. 输入格式 A single integer $n$. 输出格式 A single integer representing the number of distinct binary strings of length $n$ without consecutive $1$'s. 输入样例 2 输出样例 3