#include using namespace std; typedef long long ll; #define REP(i,n) for(int i=0,_n=(int)(n);i<_n;++i) #define ALL(v) (v).begin(),(v).end() #define CLR(t,v) memset(t,(v),sizeof(t)) templateostream& operator<<(ostream& os,const pair&a){return os<<"("<void chmin(T&a,const T&b){if(a>b)a=b;} templatevoid chmax(T&a,const T&b){if(avoid pv(T a,T b){for(T i=a;i!=b;++i)cerr<<(*i)<<" ";cerr<void pv(T a,T b){} #endif ll nextLong() { ll x; scanf("%lld", &x); return x;} int main2() { int N = nextLong(); int M = nextLong(); int A = nextLong(); int B = nextLong(); vector> g(N+3); REP(i, M) { int L = nextLong(); int R = nextLong(); g[L].push_back(R+1); g[R+1].push_back(L); } for (int a = 1; a <= A; a++) g[0].push_back(a); for (int b = B+1; b <= N+1; b++) g[b].push_back(N+2); const int INF = 1001001001; vector memo(g.size(), INF); queue q; q.push(0); memo[0] = 0; while (!q.empty()) { int cur = q.front(); q.pop(); for (int nxt : g[cur]) { if (memo[nxt] > memo[cur] + 1) { memo[nxt] = memo[cur] + 1; q.push(nxt); } } } int ans = memo[N+2] - 2; if (ans > INF / 2) ans = -1; cout << ans << endl; return 0; } int main() { #ifdef LOCAL for (;!cin.eof();cin>>ws) #endif main2(); return 0; }