#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
namespace my {
//循环
#define fi(a, b) for (a ::iterator it = (b).begin(); it != (b).end(); ++it)
#define f(i, a, b) for (int i = (a); i < (b); ++i)
#define fe(i, a, b) for (int i = (a); i <= (b); ++i)
#define fr(i, a, b) for (int i = (a); i >= (b); --i)
#define ff f(i, 0, n)
#define fff fe(i, 1, n)
#define w(a) while (a--)
//输出
#define ci(a) cin >> a;
#define co(a) cout << a;
#define ce(a) cout << a << endl;
#define c1(a) cout << a << " ";
#define c2(a, b) cout << a << " " << b << " ";
#define c3(a, b, c) cout << a << " " << b << " " << c;
//其他
#define m(a, b) memset((a), (b), sizeof((a)));
#define maxn 1000005
#define maxnn 1002
#define inf 1000000007
#define int long long
#define all(s) (s).begin(), (s).end()
#define mp make_pair
#define pb push_back
#define fir first
#define sec second
#define sp system("pause");
}  // namespace my
using namespace my;
int n, m, t, i = 0, j = 0, k = 0, a = 0, b = 0, c = 0, d = 0, sum = 0, num = 0, ans = 0;
int dp[maxn], data[maxn], save[maxn], is[maxn];
int datas[maxnn][maxnn];
int dx[8] = { 1, -1, 0, 0, 1, -1, 1, -1 }, dy[8] = { 0, 0, -1, 1, 1, 1, -1, -1 };  //下上左右
char ch, str[maxn], chess[maxnn][maxnn];
string s, ss;
int ok = 1;
deque<int> de;
queue<int> qu;
set<int> se;
vector<int> E[maxn];
vector<int> v2;
map<int, int> ma;
map<int, int> ma1;
bool dis[maxn];
pair<int, int> pa[maxn];

//#define debug
#ifdef debug
#define fre freopen("in.txt", "r", stdin);
#define d(a) cout << #a << "--" << a << "--" << endl;
#define ds(a)                                \
    cout << #a << "--" << a << "--" << endl; \
    system("pause");
#define dm(a) \
    fe(i, 0, n) { fe(j, 0, n) c1((a)[i][j]) ce("") }
#define dv(a) f(i, 0, 11) c1((a)[i]) ce("")
#else
#define d(a)
#define ds(a)
#define fre
#define da(a)
#define dv(a)
#define dm(a)
#endif
/*
void spfa(int u){
        for(int i=0;i<E[u].size() ;++i){
                if(dp[E[u][i]]>dp[u]+1) dp[E[u][i]]=dp[u]+1,spfa(E[u][i]);
        }
}
*/
signed main() {
    ios::sync_with_stdio(0);
    priority_queue<pair<int, int> > p;
    fre cin >> n >> m >> a >> b;
    w(m) {
        cin >> c >> d;
        E[c].pb(d);
        E[d].pb(c);
    }
    fff dp[i] = inf;
    dp[a] = 1;
    p.push(mp(1, a));
    while (!p.empty()) {
        int u = p.top().sec;
        p.pop();
        for (int i = 0; i < E[u].size(); ++i) {
            if (dp[E[u][i]] > dp[u] + 1) {
                dp[E[u][i]] = dp[u] + 1;
                p.push(mp(-dp[E[u][i]], E[u][i]));
            }
        }
    }
    if (dp[b] < inf)
        ce(dp[b]) else ce(-1) return 0;
}
/*
1 1 1 1
1 1
1
*/