//Author: Kevin
#include<bits/stdc++.h>
//#pragma GCC optimize("O2")
using namespace std;
#define ll long long
#define ull unsigned ll
#define pb emplace_back
#define mp make_pair
#define ALL(x) (x).begin(),(x).end()
#define rALL(x) (x).rbegin(),(x).rend()
#define srt(x) sort(ALL(x))
#define rev(x) reverse(ALL(x))
#define rsrt(x) sort(rALL(x))
#define sz(x) (int)(x.size())
#define inf 0x3f3f3f3f
#define pii pair<int,int>
#define lb(v,x) (int)(lower_bound(ALL(v),x)-v.begin())
#define ub(v,x) (int)(upper_bound(ALL(v),x)-v.begin())
#define uni(v) v.resize(unique(ALL(v))-v.begin())
#define longer __int128_t
void die(string S){puts(S.c_str());exit(0);}
vector<int> G[100100];
int dfn[100100],tot,low[100100];
bool flag[100100];
int dist1[100100],dist2[100100];
void dfs(int u,int fa=0)
{
	dfn[u]=low[u]=++tot;
	flag[u]=1;
	for(auto v:G[u])
		if(v!=fa)
		{
			if(dfn[v])
				low[u]=min(low[u],dfn[v]);
			else
			{
				dfs(v,u);
				low[u]=min(low[u],low[v]);
				if(low[v]!=dfn[v])
					flag[u]=0;
			}
		}
	if(low[u]!=dfn[u])
		flag[u]=0;
}
int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	int t;
	cin>>t;
	for(int tc=1;tc<=t;tc++)
	{
		memset(low,0,sizeof(low));
		memset(dfn,0,sizeof(dfn));
		int n,m,u,v;
		cin>>n>>m>>v>>u;
		for(int i=1;i<=n;i++)
			G[i].clear();
		for(int i=1;i<=m;i++)
		{
			int a,b;
			cin>>a>>b;
			G[a].pb(b);
			G[b].pb(a);
		}
		tot=0;
		dfs(u);
		if(!dfn[v])
		{
			printf("Case #%d: SAFE\n",tc);
			continue;
		}
		queue<int> q;
		memset(dist1,-1,sizeof(dist1));
		memset(dist2,-1,sizeof(dist2));
		q.push(u);
		dist1[u]=0;
		while(!q.empty())
		{
			int x=q.front();
			q.pop();
			for(auto y:G[x])
				if(dist1[y]==-1)
				{
					dist1[y]=dist1[x]+1;
					q.push(y);
				}
		}
		q.push(v);
		dist2[v]=0;
		while(!q.empty())
		{
			int x=q.front();
			q.pop();
			for(auto y:G[x])
				if(dist2[y]==-1)
				{
					dist2[y]=dist2[x]+1;
					q.push(y);
				}
		}
		bool espace=0;
		for(int i=1;i<=n;i++)
			if(!flag[i]&&dist2[i]<dist1[i])
				espace=1;
		if(espace)
			printf("Case #%d: SAFE\n",tc);
		else
		{
			int ans=0;
			for(int i=1;i<=n;i++)
				if(dist2[i]<dist1[i])
					ans=max(ans,dist1[i]*2);
			printf("Case #%d: %d\n",tc,ans);
		}
	}
	return 0;
}