#include <bits/stdc++.h>
#define INF 1000000001LL
#define MOD 1000000007LL
#define long long long
#define all(x) x.begin(),x.end()
using namespace std;

vector<int> graph[100003];
int main()
{
	ios_base::sync_with_stdio(0); 
    cin.tie(0);

	int n,m,a,b;
    cin >> n >> m >> a >> b;

    for(int i = 0; i<m; i++)
    {
        int x,y;
        cin >> x >> y;
        y++;
        if(x < a)
            x = a;
        if(x > b+1)
            x = b+1;
        if(y < a)
            y = a;
        if(y > b+1)
            y = b+1;
        graph[x].push_back(y);
        graph[y].push_back(x);
    }

    queue<int> q;
    vector<int> vis(n+2);

    q.push(a);
    vis[a] = 1;
    while(!q.empty())
    {
        int x = q.front();
        q.pop();

        for(int y : graph[x])
        {
            if(vis[y])
                continue;
            vis[y] = vis[x]+1;
            q.push(y);
        }
    }
    cout << vis[b+1]-1 << endl;
    return 0;
}