Problem:


Score : 1400 points

Problem Statement

Sigma and Sugim are playing a game.

The game is played on a graph with N vertices numbered 1 through N. The graph has N-1 red edges and N-1 blue edges, and the N-1 edges in each color forms a tree. The red edges are represented by pairs of integers (a_i, b_i), and the blue edges are represented by pairs of integers (c_i, d_i).

Each player has his own piece. Initially, Sigma's piece is at vertex X, and Sugim's piece is at vertex Y.

The game is played in turns, where turns are numbered starting from turn 1. Sigma takes turns 1, 3, 5, ..., and Sugim takes turns 2, 4, 6, ....

In each turn, the current player either moves his piece, or does nothing. Here, Sigma can only move his piece to a vertex that is directly connected to the current vertex by a red edge. Similarly, Sugim can only move his piece to a vertex that is directly connected to the current vertex by a blue edge.

When the two pieces come to the same vertex, the game ends immediately. If the game ends just after the operation in turn i, let i be the total number of turns in the game.

Sigma's objective is to make the total number of turns as large as possible, while Sugim's objective is to make it as small as possible.

Determine whether the game will end in a finite number of turns, assuming both players plays optimally to achieve their respective objectives. If the answer is positive, find the number of turns in the game.

Constraints

2 ≦ N ≦ 200,000

1 ≦ X, Y ≦ N

X \neq Y

1 ≦ a_i, b_i, c_i, d_i ≦ N

The N-1 edges in each color (red and blue) forms a tree.

Input

The input is given from Standard Input in the following format:

N X Y
a_1 b_1
a_2 b_2
:
a_{N-1} b_{N-1}
c_1 d_1
c_2 d_2
:
c_{N-1} d_{N-1}

Output

If the game will end in a finite number of turns, print the number of turns.
Otherwise, print -1.

Sample Input 1

4 1 2
1 2
1 3
1 4
2 1
2 3
1 4

Sample Output 1

4

Sample Input 2

3 3 1
1 2
2 3
1 2
2 3

Sample Output 2

4

Sample Input 3

4 1 2
1 2
3 4
2 4
1 2
3 4
1 3

Sample Output 3

2

Sample Input 4

4 2 1
1 2
3 4
2 4
1 2
3 4
1 3

Sample Output 4

-1

Sample Input 5

5 1 2
1 2
1 3
1 4
4 5
2 1
1 3
1 5
5 4

Sample Output 5

6



Sample input:
4 1 2
1 2
1 3
1 4
2 1
2 3
1 4


Sample output:
4

Submission status: Accepted

Solution code:
import sys
sys.setrecursionlimit(2147483647)
INF=float("inf")
MOD=10**9+7 # 998244353
input=lambda:sys.stdin.readline().rstrip()
def resolve():
    n,x,y=map(int,input().split())
    x-=1; y-=1
    E1=[[] for _ in range(n)]
    E2=[[] for _ in range(n)]
    for _ in range(n-1):
        u,v=map(int,input().split())
        u-=1; v-=1
        E1[u].append(v)
        E1[v].append(u)
    for _ in range(n-1):
        u,v=map(int,input().split())
        u-=1; v-=1
        E2[u].append(v)
        E2[v].append(u)

    # E2 において、y-rooted tree とみて、depth,par を計算
    par=[None]*n # x の必勝頂点を判定するのに必要
    par[y]=y
    depth2=[None]*n
    depth2[y]=0
    Q=[y]
    while(Q):
        v=Q.pop()
        for nv in E2[v]:
            if(depth2[nv] is not None): continue
            depth2[nv]=depth2[v]+1
            par[nv]=v
            Q.append(nv)

    # E1の辺で、E2での距離が 3 以上のものは必勝
    win=[0]*n
    for v in range(n):
        for nv in E1[v]:
            if(par[v]==nv or par[nv]==v or par[v]==par[nv] or par[par[v]]==nv or par[par[nv]]==v): continue
            win[nv]=win[v]=1

    # E1 において、x-rooted tree とみて探索
    # depth1 < depth2 -> 以降も探索できる
    # depth1 = depth2 -> そこで捕まる
    ans=depth2[x]
    depth1=[None]*n
    depth1[x]=0
    Q=[x]
    while(Q):
        v=Q.pop()
        if(win[v]): # 探索できる状態 & 必勝頂点にいれば勝ち
            print(-1)
            return
        for nv in E1[v]:
            if(depth1[nv] is not None): continue
            depth1[nv]=depth1[v]+1
            ans=max(ans,depth2[nv])
            if(depth1[nv]<depth2[nv]): Q.append(nv)

    print(2*ans)
resolve()