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说明

Do you remember that when you first entered Fujian Normal University in your freshman year, the enthusiastic seniors and sisters gave you a gift package for the start of school. There is a map in the gift package. On the map, there are $n$ teaching buildings on the map. They are connected by $m$ undirected roads, the distance of each road is $1$.

Now You are given two integers $a, b$ which refers two special teaching buildings. Please find the total number of pairs $(u,v)$ of the teaching buildings to satisfy : Every path from $u$ to $v$ must go through two special teaching buildings $a$ and $b$.

输入格式

The first line contains four integers $n,m,a,b$ $(4 \leq n \leq m \leq 10^5, 1 \leq a,b \leq n,a \neq b)$.

The next $m$ lines contains two integers $u,v$ $(1\leq u,v \leq n, u \neq v)$, represent an edge between building $u$ and $v$.

输出格式

Print the total number of pairs that meets the conditions.

样例

7 7 2 3
1 2
2 3
3 4
4 5
5 6
4 6
2 7

6