- Time limit: 1.00 s
- Memory limit: 512 MB
Input
The only input line has two integers: $n$ and $k$.
Output
Print one integer: the answer modulo $10^9+7$.
Constraints
- $1 \le k \le n \le 10^6$
Input:
3 2
Output:
5
Explanation: The valid strings are 010, 011, 101, 110 and 111.