#include "bits/stdc++.h"
using namespace std;
#define OUT(x)                cout << #x << " = " << x << endl; 
#define rep(i, n)             for (int (i) = 0; (i) < (int)(n); (i)++)
#define rer(i, l, r)          for (int (i) = (int)(l); (i) <= (int)(r); (i)++)
#define reu(i, l, r)          for (int (i) = (int)(l); (i) < (int)(r); (i)++)
#define each(i, v)            for (auto i : v)
#define all(x)                (x).begin(), (x).end()
#define rall(x)               (x).rbegin(), (x).rend()
#define pb(x)                 push_back(x)
#define bp(x)                 __builtin_popcount(x)
#define mp(x, y)              make_pair((x), (y))
#define fi                    first
#define se                    second
#define setp(x)               setprecision(x)
#define mset(m, v)            memset(m, v, sizeof(m))
#define sz(x)                 (int)(x.size())
static const int INF        = 0x3f3f3f3f;
static const long long INFL = 0x3f3f3f3f3f3f3f3fLL;
static const int MOD        = 1000000007;
static const double PI      = 3.14159265358979;

#define int                   long long

typedef vector<double>        vd;
typedef vector<string>        vs;
typedef vector<bool>          vb;
typedef vector<int>           vi;
typedef pair<int, int>        pii;
typedef vector<pii>           vpii;

template<typename T> void pv(T a, T b) { for (T i = a; i != b; i ++) cout << *i << " "; cout << endl; }
template<typename T, typename U> inline void amin(T &x, U y) { if (y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if (x < y) x = y; }
int in() { int _x; scanf("%d", &_x); return _x; }
long long lin() {long long _x; scanf("%lld", &_x); return _x; };

vpii g[101010];
int dis[101010];
int sdis[101010];
int n;

void dijkstra(int s) {
        priority_queue<pii, vector<pii>, greater<pii>> q;
        fill(dis, dis + n, INF);
        dis[s] = 0;
        q.push(mp(s, 0));
        while (!q.empty()) {
                pii p = q.top(); q.pop();
                int v = p.fi;
                if (dis[v] < p.se) continue;
                each(e, g[v]) {
                        if (dis[e.fi] > dis[v] + e.se) {
                                dis[e.fi] = dis[v] + e.se;
                                q.push(mp(e.fi, dis[e.fi]));
                        }
                }
        }
}

signed main() { 
        int m;
        cin >> n >> m;
        int s, t;
        cin >> s >> t;
        s --;
        t --;
        rep(i, m) {
                int a, b;
                cin >> a >> b;
                a --;
                b --;
                g[a].pb(mp(b, 1));
                g[b].pb(mp(a, 1));
        }
        dijkstra(s);
        rep(i, n) sdis[i] = dis[i];
        dijkstra(t);
        int scnt[101010] = { 0 };
        int tcnt[101010] = { 0 };
        rep(i, n) {
                if (sdis[i] == INF) continue;
                scnt[sdis[i]] ++;
        }
        rep(i, n) {
                if (dis[i] == INF) continue;
                tcnt[dis[i]] ++;
        }
        scnt[0] = 1;
        tcnt[0] = 1;
        int k = sdis[t];
        k -= 2;
        int ans = 0;
        rer(l, 0, k) ans += scnt[l] * tcnt[k - l];
        cout << ans << endl;
        return 0;
}               

Measured memory_bytes: 22780