#include <cstdio>
#include <climits>
#include <algorithm>
#include <queue>

const int MAXN = 100000;
const int MAXM = 1000010;

struct Node;
struct Edge;

struct Node {
	Edge *firstEdge;
	int dist;
} nodes[MAXN];

struct Edge {
	Node *from, *to;
	Edge *next;

	Edge(Node *from, Node *to) : from(from), to(to), next(from->firstEdge) {}
};

int n, m, start, t;

inline void addEdge(int u, int v) {
	nodes[u].firstEdge = new Edge(&nodes[u], &nodes[v]);
	nodes[v].firstEdge = new Edge(&nodes[v], &nodes[u]);
}

inline int bfs(int start) {
	for (int i = 0; i < n; i++) nodes[i].dist = INT_MAX;
	nodes[start].dist = 1;

	std::queue<Node *> q;
	q.push(&nodes[start]);

	while (!q.empty()) {
		Node *v = q.front();
		q.pop();

		for (Edge *e = v->firstEdge; e; e = e->next) {
			if (e->to->dist == INT_MAX) {
				e->to->dist = v->dist + 1;
				q.push(e->to);
			}
		}
	}

	int max = 0;
	for (int i = 0; i < n; i++) max = std::max(max, nodes[i].dist);

	return max;
}

int main() {
	freopen("dividecandy.in", "r", stdin);
	freopen("dividecandy.out", "w", stdout);

	scanf("%d %d %d %d", &n, &m, &start, &t);
	start--;

	for (int i = 0; i < m; i++) {
		int u, v;
		scanf("%d %d", &u, &v), u--, v--;

		addEdge(u, v);
	}

	printf("%d\n", bfs(start) + t);

	fclose(stdin);
	fclose(stdout);

	return 0;
}