#include <iostream>
#include <cmath>
#include <algorithm>
#include <vector>
#include <string>
#include <cstring>
#include <iomanip>
#include <set>

using namespace std;

vector<vector<int> > g1, g2;
int f = 0;
vector<int> used;
set<pair<int, int> > s;
int a, b;
long long sz;
vector<long long> dist1, dist2, dist3;

void dfs1(int v, int p,  long long s)
{
    //used[v] = 1;
    dist1[v] = min(dist1[v], s);
    for (int i = 0; i < g2[v].size() ; i++)
    {
        int to = g2[v][i];
        if (to != p)
            dfs1(to, v, s + 1);
    }
}

void dfs2(int v, int p, long long s)
{
    //used[v] = 1;
    dist2[v] = min(dist2[v], s);
    for (int i = 0; i < g1[v].size(); i++)
    {
        int to = g1[v][i];
        if (to != p)
            dfs2(to, v, s + 1);
    }
}

void dfs3(int v, int p, long long s)
{
    //used[v] = 1;
    dist3[v] = min(dist3[v], s);
    for (int i = 0; i < g1[v].size(); i++)
    {
        int to = g1[v][i];
        if (to != p)
            dfs3(to, v, s + 1);
    }
}

int main()
{
    int n, m;
    cin >> n >> m;
    cin >> a >> b;

    g1.resize(n + 1);
    g2.resize(n + 1);
    int x, y;
    for (int i = 0; i < m; i++)
    {
        cin >> x >> y;
        g1[y].push_back(x);
        g2[x].push_back(y);
    }

    long long ans = n * n;
    //used.assign(n + 1, 0);
    dist1.assign(n + 1, n * n);
    dist2.assign(n + 1, n * n);
    dist3.assign(n + 1, n * n);
    dfs1(0, -1, 0);
    //used.assign(n + 1, 0);
    dfs2(a, -1, 0);
    //used.assign(n + 1, 0);
    dfs3(b, -1, 0);

    for (int i = 0; i <= n; i++)
        ans = min(ans, dist1[i] + dist2[i] + dist3[i]);

    cout << ans << endl;

    return 0;
}