#include <bits/stdc++.h>
#define endl '\n'
using namespace std;

const int maxN = 1e5 + 5;

struct p
{
    int node, time;

    p(){}
    p(int node, int time)
    {
        this->node = node;
        this->time = time;
    }
};

bool vis[maxN];
int n, m, a, b, x, y;
vector<int> v[maxN];
queue<p> q;

int bfs(int beg, int en)
{
    int currNode, time;

    q.push(p(beg, 1));
    vis[beg] = true;

    bool k = false;

    while(!q.empty())
    {
        currNode = q.front().node;
        time = q.front().time;
        q.pop();
        if(currNode == en)
        {
            k = true;
            break;
        }

        for(int i = 0; i < v[currNode].size(); i++)
        {
            if(!vis[v[currNode][i]])
            {
                q.push(p(v[currNode][i], time + 1));
                vis[v[currNode][i]] = true;
            }
        }
    }

    if(!k) return -1;
    return time;
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    freopen("cities.in", "r", stdin);
    freopen("cities.out", "w", stdout);

    cin>>n>>m>>a>>b;
    for(int i = 0; i < m; i++)
    {
        cin>>x>>y;
        v[x].push_back(y);
        v[y].push_back(x);
    }

    cout<<bfs(a, b)<<endl;
    return 0;
}