#include<bits/stdc++.h>
using namespace std;
#define ALL(x) begin(x),end(x)
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define mod 1000000007
using ll=long long;
const int INF=1000000000;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0},dy[]={0,1,0,-1};
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

struct IOSetup{
    IOSetup(){
        cin.tie(0);
        ios::sync_with_stdio(0);
        cout<<fixed<<setprecision(12);
    }
} iosetup;
 
template<typename T>
ostream &operator<<(ostream &os,const vector<T>&v){
    for(int i=0;i<(int)v.size();i++) os<<v[i]<<(i+1==(int)v.size()?"":" ");
    return os;
}
template<typename T>
istream &operator>>(istream &is,vector<T>&v){
    for(T &x:v)is>>x;
    return is;
}


signed main(){
    int n,m,a,b;cin>>n>>m>>a>>b;a--,b--;
    vector<int> l(m),r(m);
    rep(i,m){
        cin>>l[i]>>r[i];
    }

    for(auto &x:l) x--;
    for(auto &x:r) x--;

    vector<vector<int>> g(n+1);
    rep(i,m){
        g[l[i]].push_back(r[i]+1);
        g[r[i]+1].push_back(l[i]);
    }

    vector<int> d(n+1,INF);
    using P=pair<int,int>;
    priority_queue<P,vector<P>,greater<P>> que;

    rep(i,a+1){
        d[i]=0;
        que.push(P(d[i],i));
    }

    while(!que.empty()){
        auto [x,now]=que.top();que.pop();
        for(auto to:g[now]){
            if(chmin(d[to],x+1)){
                que.push(P(d[to],to));
            }
        }
    }
    // debug(d);
    int res=INF;
    for(int i=b+1;i<=n;i++) chmin(res,d[i]);
    cout<<(res>=INF?-1:res)<<endl;
    return 0;
}