//public RangeAddFenwickTree seg;
public void init(HLTreeGraph G)
{
//seg = new RangeAddFenwickTree(heavy.Count + 2);
}
public void Solve()
{
var n = sc.Integer();
var a = sc.Integer() - 1;
var b = sc.Integer() - 1;
var F = new int[n - 1];
var T = new int[n - 1];
for (int i = 0; i < n - 1; i++)
{
F[i] = sc.Integer() - 1;
T[i] = sc.Integer() - 1;
}
var H = new HLTreeGraph(n);
for (int i = 0; i < n - 1; i++)
{
H.AddEdge(sc.Integer() - 1, sc.Integer() - 1);
}
H.Build(b);
var G = Enumerate(n, x => new List <int>());
var win = new bool[n];
for (int i = 0; i < n - 1; i++)
{
var f = F[i];
var t = T[i];
var d = H.Query(f, t);
if (d >= 3)
{
win[f] = win[t] = true;
}
//else
{
G[f].Add(t);
G[t].Add(f);
}
}
var D = Enumerate(n, x => H.Query(b, x) * 2);
var E = Enumerate(n, x => 1000000000);
E[a] = 0;
var q = new Queue <int>();
q.Enqueue(a);
while (q.Count > 0)
{
var p = q.Dequeue();
if (win[p])
{
IO.Printer.Out.WriteLine(-1);
return;
}
foreach (var to in G[p])
{
var d = E[p] + 2;
if (D[to] > d && E[to] > d)
{
E[to] = d;
q.Enqueue(to);
}
}
}
Debug.WriteLine(D.AsJoinedString());
Debug.WriteLine(E.AsJoinedString());
var max = 0L;
for (int i = 0; i < n; i++)
{
if (E[i] < 1000000)
{
max = Math.Max(max, D[i]);
}
}
IO.Printer.Out.WriteLine(max);
}
