#include <iostream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstdlib>
#include <cassert>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <bitset>
#include <string>
#include <algorithm>
#include <utility>
#define llint long long
#define inf 1e18
#define rep(x, s, t) for(llint (x) = (s); (x) < (t); (x)++)
#define Rep(x, s, t) for(llint (x) = (s); (x) <= (t); (x)++)
#define chmin(x, y) (x) = min((x), (y))
#define chmax(x, y) (x) = max((x), (y))

using namespace std;
typedef pair<llint, llint> P;

llint n, m, a, b;
vector<llint> G[100005];
bool used[100005], used2[100005], par[100005];

llint dfs(int v, int p)
{
	used[v] = true;
	llint ret = 0;
	for(int i = 0; i < G[v].size(); i++){
		llint u = G[v][i];
		if(u == p) continue;
		ret++;
		par[v] = !par[v], par[u] = !par[u];
		if(!used[u]) ret += dfs(u, v);
	}
	return ret;
}

vector<llint> vec;
void dfs2(int v)
{
	used2[v] = true;
	if(par[v]) vec.push_back(v);
	for(int i = 0; i < G[v].size(); i++){
		if(used2[G[v][i]]) continue;
		dfs2(G[v][i]);
	}
}

int main(void)
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	cin >> n >> m >> a >> b;
	llint l, r;
	for(int i = 1; i <= m; i++){
		cin >> l >> r;
		G[l-1].push_back(r);
		G[r].push_back(l-1);
	}
	
	llint ans = inf;
	for(int i = 0; i < a; i++){
		if(used[i]) continue;
		llint res = dfs(i, -1);
		vec.clear(), dfs2(i);
		if(vec.size() != 2) continue;
		sort(vec.begin(), vec.end());
		if(vec[0] < a && vec[1] >= b) ans = min(ans, res);
	}
	if(ans > inf/2) cout << -1 << endl;
	else cout << ans << endl;
	
	return 0;
}