#pragma GCC optimize(2)
#pragma GCC optimize(3)
#include<bits/stdc++.h>
#define int long long
#define fi first
#define se second
#define endl '\n'
#define rep(i,a,b) for(int i=a;i<=b;i++)
using namespace std;
typedef long long ll;
typedef pair<int,int> PII;
const int INF=0x3f3f3f3f,N=2e5+10;
//求从根节点r遍历一个树的时间 2*(n-1)-d d为以r为根节点的树的深度(离r最远d点)
void solve()
{
    int n;
    cin>>n;
    int a,b;
    cin>>a>>b;
    vector<vector<int>>g(n+1);
    for(int i=0;i<n-1;i++)
    {
        int u,v;
        cin>>u>>v;
        g[u].push_back(v);
        g[v].push_back(u);
    }
    vector<int>path,pp;
    auto dfs=[&](auto &&dfs,int u,int p)->void 
    {
        path.push_back(u);
        if(u==b)
        {
            pp=path;
        }
        for(auto v:g[u])
        {
            if(v==p)continue;
            dfs(dfs,v,u);
        }
        path.pop_back();
    };
    dfs(dfs,a,-1);
    int p1=0,p2=pp.size()-1;
    int ans=0;
    while(p1<p2)//求a和b的中点
    {
        p1++,p2--;
        ans++;
    }
    auto dfs1=[&](auto &&dfs1,int u,int p)->int 
    {
        int mx=0;
        for(auto v:g[u])
        {
            if(v==p)continue;
            mx=max(mx,dfs1(dfs1,v,u)+1);
        }  
        return mx;
    };
    cout<<ans+2*(n-1)-dfs1(dfs1,pp[p2],-1)<<endl;
}
signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(0),cout.tie(0);
    int t=1;
    cin>>t;
    while(t--)
    {
       solve();
    }
    return 0;
}