Ikta has an extraordinary feeling for undirected graphs. Ikta likes the undirected graph G and its two-point s, t pair (G, s, t) that has the greatest "beauty". The "beauty" of a pair (G, s, t) is the edge e = \\ {u, v \\} (u and v are two different points of G), and the shortest path from s to t in G. Is the number of things whose length is greater than the length of the shortest path from s to t in the undirected graph with g plus e.

Your job is to write a program that seeks its "beauty" given a pair (G, s, t).



Input

The input is given in the following format.

> N M s t
> x1 y1
> ...
> xi yi
> ...
> xM yM
>

First, the number of vertices, the number of edges, and the integers N, M, s, t representing the two vertices of the undirected graph are input. From the 2nd line to the M + 1th line, two vertices connected by edges are input. (However, let the set of G vertices be \\ {1, ..., N \\}.)

Constraints

Each variable being input satisfies the following constraints.

* 2 ≤ N ≤ 100,000
* 1 ≤ M ≤ 300,000
* 1 ≤ s, t, xi, yi ≤ N
* Different from s and t
* Guaranteed to reach t from s

Output

When the given graph is G, output the "beauty" of the set (G, s, t) in one line.

Examples

Input

3 2 1 3
1 2
2 3


Output

1


Input

9 8 7 8
2 6
4 9
8 6
9 2
3 8
1 8
8 5
7 9


Output

7


Input

4 3 1 4
1 2
3 4
4 1


Output

0


Input

9 7 8 9
9 6
6 5
3 6
3 7
2 5
8 5
1 4


Output

2

#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cmath>
#include<cctype>
#include<math.h>
#include<string>
#include<string.h>
#include<stack>
#include<queue>
#include<vector>
#include<utility>
#include<set>
#include<map>
#include<stdlib.h>
#include<iomanip>

using namespace std;

#define ll long long
#define ld long double
#define EPS 0.0000000001
#define INF 1e9
#define LINF (ll)INF*INF
#define MOD 1000000007
#define rep(i,n) for(int i=0;i<(n);i++)
#define loop(i,a,n) for(int i=a;i<(n);i++)
#define all(in) in.begin(),in.end()
#define shosu(x) fixed<<setprecision(x)

#define int ll //!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

typedef vector<int> vi;
typedef vector<string> vs;
typedef pair<int,int> pii;
typedef vector<pii> vp;

int gcd(int a, int b){
    if(b==0) return a;
    return gcd(b,a%b);
}
int lcm(int a, int b){
    return a*b/gcd(a,b);
}

struct edge { int to, cost;};

class Dijkstra{
private:
    vector< vector<edge> > g;

public:
    Dijkstra(){};
    Dijkstra(int size){
        g.resize(size);
    }

    vi dist;

    void add(int a, int b, int c){//無向辺
        edge e1 = {b, c}, e2 = {a, c};
        g[a].push_back(e1);
        g[b].push_back(e2);//有向辺の場合はここをコメントアウトする
    }

    void calc(int s){
        priority_queue< pii, vector<pii>, greater<pii> > q;
        int n = g.size();
        dist = vi (n, INF);
        dist[s] = 0;
        q.push(pii(0, s));

        while(!q.empty()){
            pii p = q.top();
            q.pop();
            int v = p.second;
            if(dist[v] < p.first)continue; //更新されていたら何もしない
            for(int i = 0; i < g[v].size(); i++){
                edge e = g[v][i];
                if(dist[e.to] > dist[v] + e.cost){
                    dist[e.to] = dist[v] + e.cost;
                    q.push(pii(dist[e.to], e.to));
                }
            }
        }
    }

};

signed main(void) {
    int i,j;
    int n,m,s,t;
    cin >> n >> m >> s >> t;
    s--,t--;
    Dijkstra dij = Dijkstra(n);
    rep(i,m){
        int x,y;
        cin >> x >> y;
        x--,y--;
        dij.add(x,y,1);
    }
    dij.calc(s);
    int dist = dij.dist[t] - 2;
    map<int,int> ma1;
    rep(i,n)ma1[dij.dist[i]]++;
    dij.calc(t);
    map<int,int> ma2;
    rep(i,n)ma2[dij.dist[i]]++;
    int ans = 0;
    for(auto itr = ma1.begin(); itr != ma1.end(); itr++){
        int x = itr->first;//きょり
        int a = itr->second;//個数
        int b = ma2[dist-x];//個数
        ans += a*b;
    }
    cout << ans << endl;
}

