#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
 
#if __cplusplus >= 201103L
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>
#endif
 
// C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
 
#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#endif
 
#define int long long
using namespace std;

const int MAXN = 1e5 + 5;
const int INF = 1e18 + 7;

int n, m, a, b;
vector<int> adj[MAXN];
int par[MAXN];
int sz[MAXN];
int dists[MAXN][2];
int indeg[MAXN];

void init() {
    for(int i = 1; i <= n; ++i) {
        par[i] = i; sz[i] = 1;
    }
}

int find(int u) {
    if(par[u] == u) return u;
    return par[u] = find(par[u]);
}

bool merge(int u, int v) {
    u = find(u); v = find(v);
    if(u == v) return false;
    if(sz[u] > sz[v]) swap(u, v);
    par[u] = v; sz[v] += sz[u];
    return true;
}

void bfs(int start, int type) {
    dists[start][type] = 0;
    queue<int> q; q.push(start);
    while(q.size()) {
        int u = q.front(); q.pop();
        
        for(auto v : adj[u]) {
            if(dists[v][type] == INF) {
                dists[v][type] = dists[u][type] + 1;
                q.push(v);
            }
        }
    }
}

void solve(int t) {
    cin >> n >> m >> a >> b;
    init();
    for(int i = 0; i <= n; ++i) {
        indeg[i] = 0; dists[i][0] = dists[i][1] = INF;
        adj[i].clear();
    }
    
    for(int i = 1; i <= m; ++i) {
        int x, y; cin >> x >> y;
        adj[x].push_back(y); adj[y].push_back(x);
        ++indeg[x]; ++indeg[y];
        merge(x, y);
    }
    
    if(find(a) != find(b)) {
        cout << "Case #" << t << ": SAFE\n"; return;
    }
    
    bfs(a, 0); bfs(b, 1);
    queue<int> q;
    for(int i = 1; i <= n; ++i) {
        if(indeg[i] == 1) q.push(i);
    }
    
    while(q.size()) {
        int u = q.front(); q.pop();
        
        for(auto v : adj[u]) {
            --indeg[v];
            if(indeg[v] == 1) {
                q.push(v);
            }
        }
    }
    
    for(int i = 1; i <= n; ++i) {
        if(indeg[i] > 1 && dists[i][0] < dists[i][1]) {
            cout << "Case #" << t << ": SAFE\n"; return;
        }
    }
    
    int ans = 0;
    for(int i = 1; i <= n; ++i) {
        if(dists[i][0] < dists[i][1]) {
            ans = max(ans, dists[i][1]);    // choose furthest place for her to get captured
        }
    }
    cout << "Case #" << t << ": " << 2 * ans << endl;
}

signed main() {
    int t, tot; cin >> t; tot = t;
    while(t--) solve(tot - t);
    return 0;
}