#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
using namespace std;

int INF = 1e+9;
int n, m, a, b;
int l, r;
vector<int> et[100001];
int dp[100001];

signed main() {
	int i;
	
	cin >> n >> m >> a >> b;
	for (i = 0; i < m; i++) {
		cin >> l >> r;
		et[l - 1].push_back(r);
		et[r].push_back(l - 1);
	}
	
	for (i = 0; i < n + 1; i++) {
		dp[i] = INF;
	}
	
	queue<int> que;
	for (i = 0; i < a; i++) {
		dp[i] = 0;
		que.push(i);
	}
	
	while (!que.empty()) {
		int v = que.front();
		que.pop();
		for (i = 0; i < et[v].size(); i++) {
			int nv = et[v][i];
			if (dp[nv] == INF) {
				dp[nv] = dp[v] + 1;
				que.push(nv);
			}
		}
	}
	
	int ans = INF;
	for (i = b; i <= n; i++) {
		ans = min(ans, dp[i]);
	}

	if (ans == INF) cout << -1 << endl;
	else cout << ans << endl;
	return 0;
}