#include <iostream>
#include <vector>
#include <queue>

using namespace std;

const int NMAX = 1e5 + 2;
vector<int> adj[NMAX];
bool viz[NMAX];
int dist[NMAX];

void bfs(int source)
{
    queue <int> q;
    q.push(source);
    dist[source]=0;
    viz[source]=1;

    while(!q.empty())
    {
        int nod=q.front();
        q.pop();
        for(auto& to:adj[nod])
        {
            if(!viz[to])
                q.push(to), dist[to]=dist[nod]+1;
        }
    }
}

int main()
{
    freopen("sate.in", "r", stdin);
	freopen("sate.out", "w", stdout);
	int n , m; cin >> n >> m;
	int i, a, b;
	cin>>a>>b;
	for (int i = 1; i <= m; i++)
	{
		int x, y; cin >> x >> y;
		adj[x].push_back(y);
		adj[y].push_back(x);
	}
    for(i=1;i<=n;++i)
        dist[i]=-1;
    bfs(1);
    cout<<dist[b]-dist[a-1];
    return 0;
}