# Fight Against Traffic

## 输入输出格式

### 输入格式

The firt line of the input contains integers \$ n \$ , \$ m \$ , \$ s \$ and \$ t \$ ( \$ 2<=n<=1000 \$ , \$ 1<=m<=1000 \$ , \$ 1<=s,t<=n \$ , \$ s≠t \$ ) — the number of junctions and the number of roads in Nsk, as well as the indices of junctions where mayors home and work are located respectively. The \$ i \$ -th of the following \$ m \$ lines contains two integers \$ u_{i} \$ and \$ v_{i} \$ ( \$ 1<=u_{i},v_{i}<=n \$ , \$ u_{i}≠v_{i} \$ ), meaning that this road connects junctions \$ u_{i} \$ and \$ v_{i} \$ directly. It is guaranteed that there is a path between any two junctions and no two roads connect the same pair of junctions.

### 输出格式

Print one integer — the number of pairs of junctions not connected by a direct road, such that building a road between these two junctions won't decrease the distance between junctions \$ s \$ and \$ t \$ .

## 输入输出样例

### 输入样例 #1

``````5 4 1 5
1 2
2 3
3 4
4 5
``````

### 输出样例 #1

``````0
``````

### 输入样例 #2

``````5 4 3 5
1 2
2 3
3 4
4 5
``````

### 输出样例 #2

``````5
``````

### 输入样例 #3

``````5 6 1 5
1 2
1 3
1 4
4 5
3 5
2 5
``````

### 输出样例 #3

``````3
``````