#include<bits/stdc++.h>
using namespace std;
using ll = long long;

int main(){
    int n,m,a,b;
    cin>>n>>m>>a>>b;
    vector<vector<int>> g(n+2);
    a--;b--;
    for(int i = 0;i<m;i++){
        int l,r;
        cin>>l>>r;
        l--;
        g[l].push_back(r);
        g[r].push_back(l);
    }
    int ans = 1e9;
    vector<int> vis(n+2,1e9);
    for(int i = 0;i<n;i++){
        if(vis[i]<=1e8) continue;
        deque<int> que;
        que.push_back(i);
        vector<int> use;
        vis[i] = 0;
        while(que.size()){
            int ni = que.front();
            que.pop_front();
            use.push_back(ni);
            for(auto&to:g[ni]){
                if(vis[to]<=vis[ni]+1) continue;
                vis[to] = vis[ni]+1;
                que.push_back(to);
            }
        }
        for(auto&j:use) vis[j] = 1e9;
        for(auto&j:use){
            if(j<=a) {
                que.push_back(j);
                vis[j] = 1;
            }
        }
        while(que.size()){
            int ni = que.front();
            que.pop_front();
            for(auto&to:g[ni]){
                if(vis[to]<=vis[ni]+1) continue;
                vis[to] = vis[ni] + 1;
                que.push_back(to);
            }
        }
        for(auto&j:use){
            if(j==n+1) continue;
            if(j<=b) continue;
            ans = min(ans,vis[j]-1);
        }
        for(auto&j:use) vis[j] = 0;
    }

    if(ans>=1e8) cout<<-1<<endl;
    else cout<<ans<<endl;
}