#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1);
const double eps = 1e-9;
const int inf = 2000000000;
const long long infLL = 9000000000000000000;
int dx[] = {0, 0, +1, -1};
int dy[] = {+1, -1, 0, 0};
template <typename first, typename second>
ostream &operator<<(ostream &os, const pair<first, second> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "{";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin()) os << ", ";
    os << *it;
  }
  return os << "}";
}
template <typename T>
ostream &operator<<(ostream &os, const set<T> &v) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin()) os << ", ";
    os << *it;
  }
  return os << "]";
}
template <typename T>
ostream &operator<<(ostream &os, const multiset<T> &v) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin()) os << ", ";
    os << *it;
  }
  return os << "]";
}
template <typename first, typename second>
ostream &operator<<(ostream &os, const map<first, second> &v) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if (it != v.begin()) os << ", ";
    os << it->first << " = " << it->second;
  }
  return os << "]";
}
void faltu() { cerr << '\n'; }
template <typename T>
void faltu(T a[], int n) {
  for (int i = 0; i < n; ++i) cerr << a[i] << ' ';
  cerr << '\n';
}
template <typename T, typename... hello>
void faltu(T arg, const hello &...rest) {
  cerr << arg << ' ';
  faltu(rest...);
}
const int mx = 1e3 + 1;
vector<int> adj[mx];
bool adjMatrix[mx][mx];
int distS[mx], distT[mx];
void bfs(int s, int *dist) {
  queue<int> q;
  q.push(s);
  dist[s] = 0;
  while (!q.empty()) {
    int u = q.front();
    q.pop();
    for (auto &x : adj[u]) {
      if (x != s && dist[x] == 0) {
        dist[x] = dist[u] + 1;
        q.push(x);
      }
    }
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int n, m, s, t, u, v;
  cin >> n >> m >> s >> t;
  for (int i = 1; i <= m; i++) {
    cin >> u >> v;
    adj[u].push_back(v);
    adj[v].push_back(u);
    adjMatrix[u][v] = 1;
    adjMatrix[v][u] = 1;
  }
  bfs(s, distS);
  bfs(t, distT);
  int mn = distS[t], ans = 0;
  for (int i = 1; i < n; i++) {
    for (int j = i + 1; j <= n; j++) {
      if (!adjMatrix[i][j]) {
        if (distS[i] + 1 + distT[j] >= mn && distS[j] + 1 + distT[i] >= mn)
          ans++;
      }
    }
  }
  cout << ans << '\n';
  return 0;
}
