#include <bits/stdc++.h>
#define int long long
using namespace std;
const int MAXN = 100000;
vector<vector<int> > adj(MAXN);

void solve()
{
	int n, m, s, t;
	cin >> n >> m >> s >> t;
	int distance_s[n + 1];
	int distance_t[n + 1];
	int visited[n + 1];
	memset(visited, 0, sizeof(visited));
	for (int i = 1; i <= m; ++i)
	{
		int u, v;
		cin >> u >> v;
		adj[u].push_back(v);
		adj[v].push_back(u);
	}
	for (int i = 1; i <= n; ++i)
		sort(adj[i].begin(), adj[i].end());
	queue<int> pq;
	pq.push(s);
	// Shortest Distance from S
	distance_s[s] = 0;
	while (!pq.empty())
	{
		int u = pq.front();
		pq.pop();
		visited[u] = true;
		for (int v : adj[u])
		{
			if (!visited[v])
			{
				distance_s[v] = distance_s[u] + 1;
				pq.push(v);
			}
		}
	}
	// Shortest Distance from T
	memset(visited, 0, sizeof(visited));
	pq.push(t);
	distance_t[t] = 0;
	while (!pq.empty())
	{
		int u = pq.front();
		pq.pop();
		visited[u] = true;
		for (int v : adj[u])
		{
			if (!visited[v])
			{
				distance_t[v] = distance_t[u] + 1;
				pq.push(v);
			}
		}
	}
	// for (int i = 1; i <= n; ++i)
	// 	cout << distance_s[i] << " ";
	// cout << endl;
	// for (int i = 1; i <= n; ++i)
	// 	cout << distance_t[i] << " ";
	// cout << endl;
	// cout << distance_t[s] << endl;
	// cout << distance_s[t] << endl;

	// Iterating over all vertices
	int ans = 0;
	for (int u = 1; u <= n - 1; ++u)
	{
		for (int v = u + 1; v <= n; ++v)
		{
			if (count(adj[u].begin(), adj[u].end(), v))
				continue;// u,v has a direct edge
			else
			{
				//cout << i << "   " << j << endl;
				if ((distance_s[u] + distance_t[v] + 1 < distance_s[t]) || (distance_s[v] + distance_t[u] + 1 < distance_s[t]))
					continue;
				else
					ans++;
			}
		}
	}
	cout << ans  << endl;

}

int32_t main()
{
	ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#ifndef ONLINE_JUDGE
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#endif
	//seive();
	int tc = 1;
	// cin >> tc;
	while (tc--)
		solve();

}