Hamid wrote $3014$ lines of code for the a+b problem and gave $10$ lines of it to Hamed; then the following problem came to Hamed's mind.
You are given two integers $n$ and $m$.
An array $a_1,a_2,\ldots,a_n$ is called snake if and only if all of the following conditions hold:
- All elements in
$a$are integers between$0$and$m$; $a_1 + a_2 + \cdots + a_n = m$;$a_n=\max\left([a_1, a_2, \ldots, a_n]\right)$.
We define $f(a)$ as in the following pseudocode:
function f(array a):
pos := 1
res := 0
let nxt[x] be an array such that nxt[x] is the smallest index y such that
y > x and a[y] > a[x],
or undefined if no such y exists
while pos < n:
if a[pos] < a[n]:
res += a[nxt[pos]] - a[pos]
pos := nxt[pos]
else:
pos += 1
return res
Find the sum of $f(a)$ over all snake arrays $a_1,a_2,\ldots,a_n$, modulo $10^9+7$.
Input
Each test contains multiple test cases. The first line contains the number of test cases $t$ ($1 \le t \le 10^4$). The description of the test cases follows.
The first line of each test case contains two integers $n$ and $m$ ($2 \leq n \leq 2 \cdot 10^5$, $0 \leq m \leq 2 \cdot 10^5$) — the length of a snake array and the sum of all elements in a snake array.
It is guaranteed that the sum of $m$ over all test cases does not exceed $2 \cdot 10^5$.
Output
For each test case, output a single integer — the sum of $f(a)$ over all snake arrays $a_1,a_2,\ldots,a_n$, modulo $10^9+7$.
Note
In the first test case, there are three snake arrays:
$f([0, 5]) = 5$;$f([1, 4]) = 3$;$f([2, 3]) = 1$.
Thus, the answer is $5+3+1=9$.
In the second test case, there are six snake arrays:
$f([0, 0, 4]) = 4$;$f([0, 1, 3]) = 3$;$f([1, 0, 3]) = 2$;$f([1, 1, 2]) = 1$;$f([0, 2, 2]) = 2$;$f([2, 0, 2]) = 2$.
Thus, the answer is $4+3+2+1+2+2=14$.
In the fifth test case, a possible snake array is:
$f([3, 1, 4, 1, 5, 9]) = 6$.
样例
8
2 5
3 4
4 6
5 10
6 23
100 100
142857 33333
200000 0
9
14
76
985
142112
771227753
865580631
0
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