#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1e5 + 7;
int  n, m, s, t;
long long dist[3][MAXN];
vector <int>a[MAXN];
queue <pair<int, int>> q;

int main(){
    ios_base::sync_with_stdio(0);
    cout.tie(0);
    cin.tie(0);
    cin >> n >> m >> s >> t;
    for(int i = 1; i <= m; i++){
        int x, y;
        cin >> x >> y;
        a[x].push_back(y);
        a[y].push_back(x);
    }
    for(int i = 1; i <= n; i++) dist[0][i] = dist[1][i] = INT_MAX;
    q.push({0, s});
    dist[0][s] = 0;
    while(!q.empty()){
        int u = q.front().second;
        int d_u = q.front().first;
        q.pop();
        for(auto v : a[u]){
            if(d_u % 2 == 0){
                if(dist[1][v] > dist[0][u] + 1){
                    dist[1][v] = dist[0][u] + 1;
                    q.push({dist[1][v], v});
                }
            }
            else if(d_u % 2 != 0){
                if(dist[0][v] > dist[1][u] + 1){
                    dist[0][v] = dist[1][u] + 1;
                    q.push({dist[0][v], v});
                }
            }
        }
    }
    cout << (dist[0][t] == INT_MAX ? -1 : dist[0][t] / 2);
}