#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <deque>
#include <array>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <fstream>
#include <complex>
#include <cstring>
#include <unordered_map>
#include <unordered_set>
using namespace std;

using ll = long long;
constexpr int INF = 1001001001;
constexpr int mod = 1000000007;
// constexpr int mod = 998244353;

template<class T>
inline bool chmax(T& x, T y){
    if(x < y){
        x = y;
        return true;
    }
    return false;
}
template<class T>
inline bool chmin(T& x, T y){
    if(x > y){
        x = y;
        return true;
    }
    return false;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int N, M, A, B, L, R;
    cin >> N >> M >> A >> B;
    --A;
    vector<vector<int>> g(N + 1);
    for(int i = 0; i < M; ++i){
        cin >> L >> R;
        L = max(L - 1, A);
        chmin(R, B);
        g[L].emplace_back(R);
        g[R].emplace_back(L);
    }
    vector<int> dist(N + 1, INF);
    queue<int> que;
    dist[A] = 0;
    que.emplace(A);
    while(!que.empty()){
        int from = que.front();
        que.pop();
        for(int to : g[from]){
            if(chmin(dist[to], dist[from] + 1)){
                que.emplace(to);
            }
        }
    }
    int ans = dist[B];
    if(ans == INF)  ans = -1;
    cout << ans << endl;
    
    return 0;
}