#include "bits/stdc++.h" using namespace std; using ll=long long; using vll=vector< ll>; using vvll=vector< vll>; using vvvll=vector< vvll>; using vvvvll=vector; using dd=double; using vdd=vector< dd>; using vvdd=vector< vdd>; using pll=pair; using tll=tuple; using qll=tuple; constexpr ll INF = 1LL << 60; struct Fast{ Fast(){ cin.tie(0); ios::sync_with_stdio(false); cout<::max_digits10); } } fast; #define REPS(i, S, E) for (ll i = (S); i <= (E); i++) #define REP(i, N) REPS(i, 0, (N)-1) #define DEPS(i, S, E) for (ll i = (E); i >= (S); i--) #define DEP(i, N) DEPS(i, 0, (N)-1) #define rep(i, S, E) for (ll i = (S); i <= (E); i++) #define dep(i, E, S) for (ll i = (E); i >= (S); i--) #define each(e, v) for (auto&& e : v) #define ALL(v) (v).begin(), (v).end() #define RALL(v) (v).rbegin(), (v).rend() template inline bool chmax(T &a, T b) { if (a < b) { a = b; return true; }return false; } template inline bool chmin(T &a, T b) { if (a > b) { a = b; return true; }return false; } template inline T MaxE(vector&v,ll S,ll E){ T m=v[S]; rep(i,S,E)chmax(m,v[i]); return m; } template inline T MinE(vector&v,ll S,ll E){ T m=v[S]; rep(i,S,E)chmin(m,v[i]); return m; } template inline T MaxE(vector &v) { return MaxE(v,0,(ll)v.size()-1); } template inline T MinE(vector &v) { return MinE(v,0,(ll)v.size()-1); } template inline T Sum(vector &v,ll S,ll E){ T s=T(); rep(i,S,E)s+=v[i]; return s; } template inline T Sum(vector &v) { return Sum(v,0,v.size()-1); } template inline ll sz(T &v){ return (ll)v.size(); } inline ll CEIL(ll a,ll b){ return (a<0) ? -(-a/b) : (a+b-1)/b; } inline ll FLOOR(ll a,ll b){ return -CEIL(-a,b); } //vector用テンプレート template inline vector& operator+=(vector &a,const vector &b){ for (ll i=0; i<(ll)a.size(); i++) a[i]+=b[i]; return a; } template inline vector& operator-=(vector &a,const vector &b){ for (ll i=0; i<(ll)a.size(); i++) a[i]-=b[i]; return a; } template inline vector& operator*=(vector &a,const vector &b){ for (ll i=0; i<(ll)a.size(); i++) a[i]*=b[i]; return a; } template inline vector& operator/=(vector &a,const vector &b){ for (ll i=0; i<(ll)a.size(); i++) a[i]/=b[i]; return a; } template inline vector& operator%=(vector &a,const vector &b){ for (ll i=0; i<(ll)a.size(); i++) a[i]%=b[i]; return a; } template inline vector& operator+=(vector &a,S b){ for (T &e: a) e+=b; return a; } template inline vector& operator-=(vector &a,S b){ for (T &e: a) e-=b; return a; } template inline vector& operator*=(vector &a,S b){ for (T &e: a) e*=b; return a; } template inline vector& operator/=(vector &a,S b){ for (T &e: a) e/=b; return a; } template inline vector& operator%=(vector &a,S b){ for (T &e: a) e%=b; return a; } template inline vector operator+(const vector &a,S b){ vector c=a; return c+=b; } template inline vector operator-(const vector &a,S b){ vector c=a; return c-=b; } template inline vector operator*(const vector &a,S b){ vector c=a; return c*=b; } template inline vector operator/(const vector &a,S b){ vector c=a; return c/=b; } template inline vector operator%(const vector &a,S b){ vector c=a; return c%=b; } template inline vector operator-(S b,const vector &a){ vector c=-a; return c+=b; } template inline vector operator-(const vector &a){ vector c=a; return c*=(-1); } template inline ostream &operator<<(ostream &os,const vector &a){ for (ll i=0; i<(ll)a.size(); i++) os<<(i>0?" ":"")<; using vvpll=vector; template inline pair& operator+=(pair &a,const pair &b){ a.first+=b.first; a.second+=b.second; return a; } template inline pair& operator-=(pair &a,const pair &b){ a.first-=b.first; a.second-=b.second; return a; } template inline pair& operator*=(pair &a,const pair &b){ a.first*=b.first; a.second*=b.second; return a; } template inline pair& operator/=(pair &a,const pair &b){ a.first/=b.first; a.second/=b.second; return a; } template inline pair& operator%=(pair &a,const pair &b){ a.first%=b.first; a.second%=b.second; return a; } template inline pair& operator+=(pair &a,R b){ a.first+=b; a.second+=b; return a; } template inline pair& operator-=(pair &a,R b){ a.first-=b; a.second-=b; return a; } template inline pair& operator*=(pair &a,R b){ a.first*=b; a.second*=b; return a; } template inline pair& operator/=(pair &a,R b){ a.first/=b; a.second/=b; return a; } template inline pair& operator%=(pair &a,R b){ a.first%=b; a.second%=b; return a; } template inline pair operator+(const pair &a,R b){ pair c=a; return c+=b; } template inline pair operator-(const pair &a,R b){ pair c=a; return c-=b; } template inline pair operator*(const pair &a,R b){ pair c=a; return c*=b; } template inline pair operator/(const pair &a,R b){ pair c=a; return c/=b; } template inline pair operator%(const pair &a,R b){ pair c=a; return c%=b; } template inline pair operator-(R b,const pair &a){ pair c=-a; return c+=b; } template inline pair operator-(const pair &a){ pair c=a; return c*=(-1); } template inline ostream &operator<<(ostream &os,const pair &a){ return os << a.first << ' ' << a.second; } vll bfs(vvll &to, ll s){ vll dist(to.size(), INF); queue Q; dist[s] = 0; Q.push(s); while (!Q.empty()){ ll v = Q.front(); Q.pop(); each(u, to[v]) if(chmin(dist[u], dist[v]+1)) Q.push(u); } return move(dist); } void solve() { ll n,m,a,b; cin >> n >> m >> a >> b; ll bb=b-a+2;//ノード数 vvll to(bb); //vector> lr; rep(i,0,m-1){ ll l; ll r; cin>>l>>r; if (l>=b+1)continue; if (r<=a-1)continue; chmax(l,a); chmin(r,b); l-=a; r-=a; to[l].push_back(r+1); to[r+1].push_back(l); } vll dist = bfs(to, 0); ll ans=dist[bb-1]; if (ans==INF){ cout << -1 << '\n'; return; } cout << ans << '\n'; //sort(ALL(lr),[](auto &a,auto &b){return get<1>(a) < get<1>(b); }); //vll dp(bb+1,INF); //dp[0]=0; //each(e,lr){ // ll l,r; tie(l,r) = e; // chmin(dp[r],dp[l-1]+1); //} //ll ans=dp[bb]; //if (ans>=INF){ // cout << -1 << '\n'; return; //} //cout << ans << '\n'; } int main(){ #if 1 solve(); #else ll t; cin >> t; rep(i, 0, t-1){ solve(); } #endif return 0; }