public void Solve()
{
int N = Reader.Int(), A = Reader.Int() - 1, B = Reader.Int() - 1;
var E = new List <int> [2][];
for (int who = 0; who < 2; who++)
{
E[who] = new List <int> [N];
for (int i = 0; i < N; i++)
{
E[who][i] = new List <int>();
}
for (int i = 0; i < N - 1; i++)
{
int a = Reader.Int() - 1, b = Reader.Int() - 1;
E[who][a].Add(b);
E[who][b].Add(a);
}
}
var lca = new LowestCommonAncestor(B, E[1]);
var seen = new bool[N];
var que = new Queue <int>();
que.Enqueue(A);
int ans = lca.Depth(A) * 2;
for (int steps = 1; que.Count > 0; steps++)
{
var nextQue = new Queue <int>();
while (que.Count > 0)
{
int at = que.Dequeue();
foreach (int next in E[0][at])
{
if (!seen[next])
{
if (lca.Dist(at, next) > 2)
{
Console.WriteLine(-1); return;
}
seen[next] = true;
int nextDepth = lca.Depth(next);
if (nextDepth >= steps)
{
ans = Math.Max(ans, nextDepth * 2);
}
if (nextDepth > steps)
{
nextQue.Enqueue(next);
}
}
}
}
que = nextQue;
}
Console.WriteLine(ans);
}