// Copyright (C) 2017 __debug.

// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU General Public License as
// published by the Free Software Foundation; version 3

// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this program; If not, see <http://www.gnu.org/licenses/>.


#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/priority_queue.hpp>

#define x first
#define y second
#define MP std::make_pair
#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(), (x).end()
#define DEBUG(...) fprintf(stderr, __VA_ARGS__)
#ifdef __linux__
#define getchar getchar_unlocked
#define putchar putchar_unlocked
#endif

using std::pair;
using std::vector;
using std::string;

typedef long long LL;
typedef pair<int, int> Pii;

const int oo = 0x3f3f3f3f;

template<typename T> inline bool chkmax(T &a, T b) { return a < b ? a = b, true : false; }
template<typename T> inline bool chkmin(T &a, T b) { return b < a ? a = b, true : false; }
string procStatus()
{
    std::ifstream t("/proc/self/status");
    return string(std::istreambuf_iterator<char>(t), std::istreambuf_iterator<char>());
}
template<typename T> T read(T &x)
{
    int f = 1;
    char ch = getchar();
    for (; !isdigit(ch); ch = getchar())
        f = (ch == '-' ? -1 : 1);
    for (x = 0; isdigit(ch); ch = getchar())
        x = 10 * x + ch - '0';
    return x *= f;
}
template<typename T> void write(T x)
{
    if (x == 0) {
        putchar('0');
        return;
    }
    if (x < 0) {
        putchar('-');
        x = -x;
    }
    static char s[20];
    int top = 0;
    for (; x; x /= 10)
        s[++top] = x % 10 + '0';
    while (top)
        putchar(s[top--]);
}
// EOT

const int MAXN = 2e5 + 5, LOGN = 19;

int N, X, Y;
vector<int> GX[MAXN], GY[MAXN];

void input()
{
    read(N); read(X); read(Y);
    for (int i = 1; i < N; ++i) {
        int u, v;
        read(u); read(v);
        GX[u].push_back(v);
        GX[v].push_back(u);
    }
    for (int i = 1; i < N; ++i) {
        int u, v;
        read(u); read(v);
        GY[u].push_back(v);
        GY[v].push_back(u);
    }
}

int dep[MAXN], jmp[MAXN][LOGN];
int ans;

void dfsY(int u)
{
    dep[u] = dep[jmp[u][0]] + 1;
    for (int i = 0; i < SZ(GY[u]); ++i) {
        int v = GY[u][i];
        if (v == jmp[u][0])
            continue;
        jmp[v][0] = u;
        for (int j = 1; j < LOGN; ++j) {
            jmp[v][j] = jmp[jmp[v][j - 1]][j - 1];
        }
        dfsY(v);
    }
}

int getnextY(int x, int y)
{
    assert(x != y);
    for (int i = LOGN - 1; i >= 0; --i) {
        if (dep[jmp[x][i]] > dep[y])
            x = jmp[x][i];
    }
    return jmp[x][0] == y ? x : jmp[y][0];
}

int getlcaY(int u, int v)
{
    if (dep[u] < dep[v])
        std::swap(u, v);
    for (int i = LOGN - 1; i >= 0; --i) {
        if (dep[jmp[u][i]] >= dep[v])
            u = jmp[u][i];
    }
    if (u == v)
        return u;
    for (int i = LOGN - 1; i >= 0; --i) {
        if (jmp[u][i] != jmp[v][i]) {
            u = jmp[u][i];
            v = jmp[v][i];
        }
    }
    return jmp[u][0];
}

int getdisY(int u, int v)
{
    return dep[u] + dep[v] - dep[getlcaY(u, v)] * 2;
}

void dfsX(int x, int y, int fa, int cur)
{
    chkmax(ans, cur + getdisY(x, y) * 2);
    if (x == y)
        return;
    for (int i = 0; i < SZ(GX[x]); ++i) {
        int v = GX[x][i];
        if (v == fa)
            continue;
        if (v == y) {
            chkmax(ans, cur + 1);
            continue;
        }
        if (getdisY(x, v) > 2) {
            puts("-1");
            exit(0);
        }
        dfsX(v, getnextY(v, y), x, cur + 2);
    }
}

void solve()
{
    dfsY(Y);
    dfsX(X, Y, 0, 0);
    printf("%d\n", ans);
}

int main()
{
#ifdef __DEBUG
    freopen("E.in", "r", stdin);
    freopen("E.out", "w", stdout);
#endif

    input();
    solve();

    return 0;
}

// 芙蓉如面柳如眉，对此如何不泪垂。
//     -- 白居易《长恨歌》


Measured memory_bytes: 51964