#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> ostream &operator<<(ostream &os, const vector<T> &as) { const int sz = as.size(); os << "["; for (int i = 0; i < sz; ++i) { if (i >= 256) { os << ", ..."; break; } if (i > 0) { os << ", "; } os << as[i]; } return os << "]"; }
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> bool chmin(T &t, const T &f) { if (t > f) { t = f; return true; } return false; }
template <class T> bool chmax(T &t, const T &f) { if (t < f) { t = f; return true; } return false; }
#define COLOR(s) ("\x1b[" s "m")


int N, M, S, T;
vector<int> A, B;

vector<vector<int>> graph;
int zeit;
vector<int> par, dis, low;
vector<int> us;

void dfs(int u, int p) {
  par[u] = p;
  dis[u] = low[u] = zeit++;
  us.push_back(u);
  for (const int v : graph[u]) if (p != v) {
    if (~dis[v]) {
      chmin(low[u], dis[v]);
    } else {
      dfs(v, u);
      chmin(low[u], low[v]);
    }
  }
}

int main() {
  for (int numCases; ~scanf("%d", &numCases); ) { for (int caseId = 1; caseId <= numCases; ++caseId) {
    scanf("%d%d%d%d", &N, &M, &S, &T);
    --S;
    --T;
    A.resize(M);
    B.resize(M);
    for (int i = 0; i < M; ++i) {
      scanf("%d%d", &A[i], &B[i]);
      --A[i];
      --B[i];
    }
    
    graph.assign(N, {});
    for (int i = 0; i < M; ++i) {
      graph[A[i]].push_back(B[i]);
      graph[B[i]].push_back(A[i]);
    }
    zeit = 0;
    par.assign(N, -1);
    dis.assign(N, -1);
    low.assign(N, -1);
    us.clear();
    dfs(S, -1);
    
    // cycle edges
    int cnt = 0;
    vector<int> dp(N, 0);
    for (int i = 0; i < M; ++i) {
      int u = A[i], v = B[i];
      if (~dis[u] && ~dis[v]) {
        if (dis[u] > dis[v]) swap(u, v);
        if (u == par[v] && dis[v] <= low[v]) {
          // bridge
// cerr<<"bridge "<<u<<" "<<v<<endl;
        } else {
          ++cnt;
          ++dp[u];
        }
      }
    }
    
    reverse(us.begin(), us.end());
    for (const int u : us) {
      if (~par[u]) {
        dp[par[u]] += dp[u];
      }
    }
    int z = -1;
    for (const int u : us) {
      if (dp[u] == cnt) {
        z = u;
        break;
      }
    }
    assert(~z);
// cerr<<"dp = "<<dp<<endl;
// cerr<<"z = "<<z<<endl;
    
    vector<int> dist[2];
    for (int h = 0; h < 2; ++h) {
      const int src = h ? T : S;
      queue<int> que;
      dist[h].assign(N, -1);
      dist[h][src] = 0;
      que.push(src);
      for (; que.size(); ) {
        const int u = que.front();
        que.pop();
        for (const int v : graph[u]) {
          if (!~dist[h][v]) {
            dist[h][v] = dist[h][u] + 1;
            que.push(v);
          }
        }
      }
// cerr<<"dist("<<src<<", *) = "<<dist[h]<<endl;
    }
    
    assert(~dist[0][z]);
    int ans = 0;
    if (!~dist[1][z] || dist[0][z] < dist[1][z]) {
      ans = -1;
    } else {
      for (int u = 0; u < N; ++u) if (~dist[0][u]) {
        if (!~dist[1][u]) {
          ans = -1;
        } else if (dist[0][u] < dist[1][u]) {
          chmax(ans, 2 * dist[1][u]);
        }
      }
    }
    
    printf("Case #%d: ", caseId);
    if (~ans) {
      printf("%d", ans);
    } else {
      printf("SAFE");
    }
    puts("");
  }
#ifndef LOCAL
  break;
#endif
  }
  return 0;
}