#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <vector>
using namespace std;

namespace solve
{
    const int maxn = 1e5 + 10;

    vector<int> e[maxn];
    inline void add(int x, int y) { e[x].push_back(y), e[y].push_back(x); }

    int deg[maxn], n, m;

    int is_leaf[maxn];
    int vis[maxn];

    void init()
    {
        static int q[maxn];
        int l = 1, r = 0;
        for (int i = 1; i <= n; i++)
            if (deg[i] == 1)
                q[++r] = i;
        while (l <= r)
        {
            int x = q[l++];
            is_leaf[x] = 1;
            for (int v : e[x])
                if (--deg[v] == 1)
                    q[++r] = v;
        }
    }

    int dis1[maxn], dis2[maxn], A, B;

    void bfs(int s, int *dis)
    {
        for (int i = 1; i <= n; i++)
            dis[i] = 1e9;
        dis[s] = 0;
        static int q[maxn];
        int l = 1, r = 0;
        q[++r] = s;
        while (l <= r)
        {
            int x = q[l++];
            for (int v : e[x])
                if (dis[v] > dis[x] + 1)
                    dis[v] = dis[x] + 1, q[++r] = v;
        }
    }

    void main(int id)
    {
        cin >> n >> m >> A >> B;
        for (int i = 1; i <= n; i++)
            deg[i] = 0, e[i].clear(), is_leaf[i] = 0, vis[i] = 0;
        for (int i = 0, x, y; i < m; i++)
            cin >> x >> y, deg[x]++, deg[y]++, add(x, y);
        init(), bfs(A, dis1), bfs(B, dis2);
        int flag = 0;
        for (int i = 1; i <= n; i++)
            if (!is_leaf[i])
                flag |= dis1[i] < dis2[i];
        int ans = 0;
        for (int i = 1; i <= n; i++)
            if (dis1[i] < dis2[i])
                ans = max(ans, dis2[i]);
        if (ans > n)
            flag = 1;
        cout << "Case #" << id << ": ";
        if (flag == 0)
            cout << ans * 2 << '\n';
        else
            cout << "SAFE" << '\n';
    }
}

int main()
{
    ios::sync_with_stdio(false), cin.tie(0);
    int T = 1;
    cin >> T;
    for (int i = 1; i <= T; i++)
        solve::main(i);
}