#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#include <time.h>
#include <stack>
#include <array>
#define popcount __builtin_popcount
using namespace std;
using ll=long long;
typedef pair<int, int> P;

int main()
{
    int n, m, a, b; cin>>n>>m>>a>>b;
    a--;
    int l[100010], r[100010];
    vector<int> g[100010];
    for(int i=0; i<m; i++){
        cin>>l[i]>>r[i];
        l[i]--;
        g[l[i]].push_back(r[i]);
        g[r[i]].push_back(l[i]);
    }
    int d[100010];
    const int INF=1e9;
    fill(d, d+n+1, INF);
    queue<int> que;
    for(int i=0; i<=a; i++){
        que.push(i);
        d[i]=0;
    }
    while(!que.empty()){
        int x=que.front(); que.pop();
        for(auto y:g[x]){
            if(d[y]>d[x]+1){
                d[y]=d[x]+1;
                que.push(y);
            }
        }
    }
    int mn=INF;
    for(int i=b; i<=n; i++){
        mn=min(mn, d[i]);
    }
    if(mn==INF) cout<<-1<<endl;
    else cout<<mn<<endl;
    return 0;
}