#include<bits/stdc++.h>
using namespace std;

#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define rep(i,n,m) for(int i = (n); i < (int)(m); i++)
#define rrep(i,n,m) for(int i = (m) - 1; i >= (int)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD = 1e9+7;
//const ll MOD = 998244353;
const ll INF = 1e9+1;
template<typename T>
bool chmin(T &a,T b){if(a>b){a=b;return true;}else return false;}
template<typename T>
bool chmax(T &a,T b){if(a<b){a=b;return true;}else return false;}
template<typename T>
vector<ll>dx={1,0,-1,0,1,1,-1,-1};
vector<ll>dy={0,1,0,-1,1,-1,1,-1};
template<typename T>
vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v(ts...))>(a,make_v(ts...));
}
int main(){
	int n,m,a,b;cin>>n>>m>>a>>b;
	a--;b--;
	vector<int>l(m),r(m);
	rep(i,0,m){
		cin>>l[i]>>r[i];
		l[i]--;r[i]--;
	}
	int res=INF;
	ll m2=min(m,60);
	rep(i,0,1LL<<m2){
		bitset<50>x=i;
		vector<int>k(n+1);
		rep(j,0,m){
			if(x[j]){
				rep(o,l[j],r[j]+1)k[o]^=1;
			}
		}
		int cnt=0;
		rep(j,0,n)if(k[j]==1&&k[j+1]==0)cnt++;
		if(cnt>=2)continue;
		if(k[a]&&k[b])chmin(res,__builtin_popcount(i));
	}
	if(res==INF)cout<<-1<<endl;
	else cout<<res<<endl;
  return 0;
}