题目描述
There is a building with $n$ rooms, numbered $1$ to $n$.
We can move from any room to any other room in the building.
Let us call the following event a **move**: a person in some room $i$ goes to another room $j$ $(i\ne j)$.
Initially, there was one person in each room in the building.
After that, we know that there were exactly $k$ moves happened up to now.
We are interested in the number of people in each of the $n$ rooms now. How many combinations of numbers of people in the $n$ rooms are possible?
Find the count modulo $(10^9+7)$.
输入格式
Input is given from Standard Input in the following format:
> $n\quad k$
#### Constraints
- All values in input are integers.
- $3\le n\le 2\times 10^5$
- $2\le k\le 10^9$
输出格式
Print the number of possible combinations of numbers of people in the $n$ rooms now, modulo $(10^9+7)$.
样例输入 #1
3 2
样例输出 #1
10
样例输入 #2
200000 1000000000
样例输出 #2
607923868
样例输入 #3
15 6
样例输出 #3
22583772
提示
***For Sample #1:***
Let $c_1$, $c_2$, and $c_3$ be the number of people in Room $1$, $2$, and $3$ now, respectively. There are $10$ possible combination of $(c_1,c_2,c_3)$:
- $(0,0,3)$
- $(0,1,2)$
- $(0,2,1)$
- $(0,3,0)$
- $(1,0,2)$
- $(1,1,1)$
- $(1,2,0)$
- $(2,0,1)$
- $(2,1,0)$
- $(3,0,0)$
For example, $(c_1,c_2,c_3)$ will be $(0,1,2)$ if the person in Room $1$ goes to Room $2$ and then one of the persons in Room $2$ goes to Room $3$.
***For Sample #2:***
Print the count modulo $(10^9+7)$.
来源
AtCoder [ABC156E]