#include <bits/stdc++.h>
#include <random>

using namespace std;

typedef unsigned long long ull;
typedef long long ll;
typedef double ld;
//#define ll __int128
//#define int ll
//#define char ll
//#define double ld
typedef vector<char> vc;
typedef vector<vc> vvc;
typedef vector<vvc> vvvc;
typedef pair<int, int> pii;
typedef pair<pii, pii> piii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<pii> vpi;
typedef vector<vi> vvi;
typedef vector<vvi> vvvi;
typedef vector<short> vs;
typedef vector<vs> vvs;
typedef vector<vvs> vvvs;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
typedef vector<ld> vld;
typedef vector<vld> vvld;
typedef vector<vvld> vvvld;
typedef vector<string> vst;
typedef vector<vst> vvst;
typedef pair<ld, ld> pld;

#define inmin(a, b) a = min(a, (b))
#define inmax(a, b) a = max(a, (b))
#define ALL(a) a.begin(),a.end()
#define RALL(a) a.rbegin(),a.rend()
#define sqr(x) ((x) * (x))
#define fori(i, n) for(int i = 0; i < int(n); ++i)
#define SZ(a) ((int)((a).size()))
#define triple(T) tuple<T, T, T>
#define quad(T) tuple<T, T, T, T>
#define watch(x) cerr << (#x) << " = " << (x) << endl;

#ifdef MAX_HOME
#define cerr cout
#else
#define cerr if (false) cerr
#endif

const double PI = 2 * acos(0.0);
#define rand shittttty_shit
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
mt19937_64 rng_64(chrono::steady_clock::now().time_since_epoch().count());

const string DIGITS = "0123456789";
const string ALPH = "abcdefghijklmnopqrstuvwxyz";


template<class T0, class T1>
inline ostream &operator<<(ostream &out, pair<T0, T1> &a) {
    return out << "{" << a.first << ", " << a.second << "}";
}

template<class T0, class T1>
inline istream &operator>>(istream &in, pair<T0, T1> &a) {
    return in >> a.first >> a.second;
}

template<class T0, class T1, class T2>
inline ostream &operator<<(ostream &out, tuple<T0, T1, T2> &a) {
    return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << "}";
}

template<class T0, class T1, class T2, class T3>
inline ostream &operator<<(ostream &out, tuple<T0, T1, T2, T3> &a) {
    return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << ", " << get<3>(a) << "}";
}

template<class T>
inline ostream &operator<<(ostream &out, vector<T> &a) {
    out << "[";
    fori (i, a.size())out << a[i] << vector<string>{", ", "]  "}[i + 1 == a.size()];
    return out;
}


void smain();


signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
#ifdef MAX_HOME
    freopen("input.txt", "r", stdin);
    clock_t start = clock();
#endif
#ifdef HK_CUP
    freopen("finput.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif

    cout << setprecision(12) << fixed;
    smain();
#ifdef MAX_HOME
#ifndef HK_CUP
    cout << "\n\nTOTAL EXECUTION TIME: " << float(clock() - start) / CLOCKS_PER_SEC << endl;
#endif
#endif
    return 0;
}

const int N = 2e5 + 10;
const int LOG = 20;

struct Tree {
    vvi g;
    vi parent;
    int timer = 1;
    vi tin, tout;
    vi dip;

    vvi dp;

    Tree(int n) {
        g.resize(n + 1);
        parent.resize(n + 1, -1);
        tin.resize(n + 1);
        tout.resize(n + 1);
        dip.resize(n + 1, 0);
        dp.resize(n + 1, vi(LOG));
    }

    void add_edge(int v, int u) {
        g[u].push_back(v);
        g[v].push_back(u);
    }

    void dfs(int v, int par = -1) {
        tin[v] = timer++;
        if (par == -1)
            par = v;
        parent[v] = par;
        dp[v][0] = par;
        for (int i = 1; i < LOG; ++i) {
            dp[v][i] = dp[dp[v][i - 1]][i - 1];
        }
        for (int to : g[v]) {
            if (to == par)
                continue;
            dip[to] = dip[v] + 1;
            dfs(to, v);
        }
        tout[v] = timer++;
    }

    int dist(int u, int v) {
        if (!is_par(u, v))
            swap(u, v);
        return dip[v] - dip[u];
    }

    int jump(int v, int dst) {
        if (!dst)
            return v;
        for (int i = LOG - 1; i >= 0; --i) {
            if ((1 << i) < dst) {
                v = dp[v][i];
                dst -= 1 << i;
            }
        }
        return dp[v][0];
    }

    bool is_par(int u, int v) {
        return tin[u] <= tin[v] && tin[v] <= tout[u];
    }
};

int n, x, y;

Tree input_tree(int n, int root) {
    Tree ret(n);
    fori (_, n - 1) {
        int u, v;
        cin >> u >> v;
        ret.add_edge(u, v);
    }
    ret.dfs(root);
    return ret;
}

const int oo = 1e9 + 10;
int dp[N];
Tree red(N), blue(N);

int find_to(int root, int to) {
    if (!blue.is_par(root, to))
        return oo;
    return blue.jump(to, blue.dist(root, to) - 1);
}

void dfs(int v) {
//    cerr << "dp[" << v << "] = " << dp[v] << endl;
    for (int to : red.g[v]) {
        if (to == red.parent[v])
            continue;
        if (blue.parent[to] == dp[v] ||
            blue.parent[dp[v]] == to ||
            to == dp[v]) {
            continue;
        }
        dp[to] = find_to(dp[v], to);
        if (dp[to] != oo) {
            dfs(to);
        }
    }
}

void smain() {
    cin >> n >> x >> y;
    red = input_tree(n, x);
    blue = input_tree(n, y);
    fill(dp, dp + n + 1, oo);
    dp[x] = y;
    dfs(x);

    for (int v = 1; v <= n; ++v) {
        if (dp[v] == oo)
            continue;
        int subt = blue.parent[v];
        for (int to : red.g[v]) {
            if (blue.is_par(subt, to) ||
                to == subt ||
                blue.parent[to] == subt ||
                blue.parent[subt] == to)
                continue;
            cout << "-1\n";
            exit(0);
        }
    }

    int ans = 0;
    for (int v = 1; v <= n; ++v) {
        if (dp[v] != oo) {
            inmax(ans, blue.dip[v]);
        }
    }
    cout << ans * 2;

}

Measured memory_bytes: 123108