// Template {{{
#include <bits/stdc++.h>
#define REP(i,n) for(int i=0; i<(int)(n); ++i)
using namespace std;
typedef long long LL;

#ifdef LOCAL
#include "contest.h"
#else
#define dump(x) 
#endif

class range {
    struct Iterator {
        int val, inc;
        int operator*() {return val;}
        bool operator!=(Iterator& rhs) {return val < rhs.val;}
        void operator++() {val += inc;}
    };
    Iterator i, n;
    public:
    range(int e) : i({0, 1}), n({e, 1}) {}
    range(int b, int e) : i({b, 1}), n({e, 1}) {}
    range(int b, int e, int inc) : i({b, inc}), n({e, inc}) {}
    Iterator& begin() {return i;}
    Iterator& end() {return n;}
};

const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
inline bool valid(int x, int w) { return 0 <= x && x < w; }

void iostream_init() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.setf(ios::fixed);
    cout.precision(12);
}
//}}}

struct Edge{ int dst, cost; };
typedef vector<Edge> Node;
typedef vector<Node> Graph;
const int INF = 1000000000;
void add_edge(Graph& G, int a, int b) {
    G[a].push_back({b, 1});
    G[b].push_back({a, 1});
}
vector<int> SPFA(const Graph& G, int s) {
    int n = G.size();
    vector<int> dist(n, INF);
    vector<bool> inque(n);
    queue<int> que;

    dist[s] = 0;
    que.push(s);
    inque[s] = true;
    while(!que.empty()){
        int v = que.front();
        que.pop();
        inque[v] = false;
        for(Edge e : G[v]) {
            if(dist[e.dst] > dist[v] + e.cost) {
                dist[e.dst] = dist[v] + e.cost;
                if(!inque[e.dst]) {
                    que.push(e.dst);
                    inque[e.dst] = true;
                }
            }
        }
    }
    return dist;
}
int main(){
    iostream_init();
    int N, M, s, t;
    while(cin >> N >> M >> s >> t) {
        s--; t--;
        Graph G(N);
        REP(i, M) {
            int a, b;
            cin >> a >> b;
            a--; b--;
            add_edge(G, a, b);
        }

        vector<int> ds = SPFA(G, s);
        vector<int> dt = SPFA(G, t);
        map<int, int> cnt;
        for(int x : dt) cnt[x]++;
        int D = ds[t];
        long long ans = 0;
        REP(i, N) {
            // ds[i] + 1 + E == D-1
            int E = D - ds[i] - 2;
            ans += cnt[E];
        }
        cout << ans << endl;

    }

    return 0;
}

/* vim:set foldmethod=marker: */