#include <bits/stdc++.h>
using namespace std;
struct fast_ios { fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
#define FOR(i, begin, end) for(int i=(begin);i<(end);i++)
#define REP(i, n) FOR(i,0,n)
#define IFOR(i, begin, end) for(int i=(end)-1;i>=(begin);i--)
#define IREP(i, n) IFOR(i,0,n)
#define Sort(v) sort(v.begin(), v.end())
#define Reverse(v) reverse(v.begin(), v.end())
#define all(v) v.begin(),v.end()
#define SZ(v) ((int)v.size())
#define Lower_bound(v, x) distance(v.begin(), lower_bound(v.begin(), v.end(), x))
#define Upper_bound(v, x) distance(v.begin(), upper_bound(v.begin(), v.end(), x))
#define Max(a, b) a = max(a, b)
#define Min(a, b) a = min(a, b)
#define bit(n) (1LL<<(n))
#define bit_exist(x, n) ((x >> n) & 1)
#define debug(x) cout << #x << "=" << x << endl;
#define vdebug(v) cout << #v << "=" << endl; REP(i_debug, v.size()){ cout << v[i_debug] << ","; } cout << endl;
#define mdebug(m) cout << #m << "=" << endl; REP(i_debug, m.size()){ REP(j_debug, m[i_debug].size()){ cout << m[i_debug][j_debug] << ","; } cout << endl;}
#define pb push_back
#define f first
#define s second
#define int long long
#define INF 1000000000000000000
template<typename T> istream &operator>>(istream &is, vector<T> &v){ for (auto &x : v) is >> x; return is; }
template<typename T> ostream &operator<<(ostream &os, vector<T> &v){ for(int i = 0; i < v.size(); i++) { cout << v[i]; if(i != v.size() - 1) cout << endl; }; return os; }
template<typename T> void Out(T x) { cout << x << endl; }
template<typename T1, typename T2> void Ans(bool f, T1 y, T2 n) { if(f) Out(y); else Out(n); }

using vec = vector<int>;
using mat = vector<vec>;
using Pii = pair<int, int>;
using PiP = pair<int, Pii>;
using PPi = pair<Pii, int>;
using bools = vector<bool>;
using pairs = vector<Pii>;

//int dx[4] = {1,0,-1,0};
//int dy[4] = {0,1,0,-1};
//char d[4] = {'D','R','U','L'};

const int mod = 1000000007;
//const int mod = 998244353;
#define Add(x, y) x = (x + (y)) % mod
#define Mult(x, y) x = (x * (y)) % mod

struct edge{int to, cost;};

class Graph
{
public:
    int N;
    vector<vector<edge>> G;
    vec par, d;

    Graph(int N): N(N){
        G = vector<vector<edge>>(N, vector<edge>(0));
        par.resize(N);
        d.resize(N);
    }

    void add_Directed_edge(int from, int to, int cost = 1){
        G[from].push_back(edge({to, cost}));
    }

    void add_Undirected_edge(int v1, int v2, int cost = 1){
        add_Directed_edge(v1, v2, cost);
        add_Directed_edge(v2, v1, cost);
    }

    void dfs(int v, int p = -1){
        par[v] = p;
        if(p == -1) d[v] = 0;
        else{
            par[v] = p;
            d[v] = d[p] + 1;
        }
        for(auto e: G[v]) if(e.to != p) dfs(e.to, v);
    }

    void calcans(int v, int &ans, vec &D, vec &Par){
        Max(ans, 2 * D[v]);
        if(D[v] <= d[v]) return;
        for(auto e: G[v]) if(e.to != par[v]){
            if(v != Par[e.to] && e.to != Par[v] && Par[v] != Par[e.to] && !(Par[v] != -1 && Par[Par[v]] == e.to) && !(Par[e.to] != -1 && Par[Par[e.to]] == v)) ans = INF;
            calcans(e.to, ans, D, Par);
        } 
    }

};

signed main(){

    int N, X, Y; cin >> N >> X >> Y;
    X--; Y--;
    Graph R(N), B(N);
    int a, b;
    REP(i, N - 1){
        cin >> a >> b;
        R.add_Undirected_edge(a - 1, b - 1);
    }
    REP(i, N - 1){
        cin >> a >> b;
        B.add_Undirected_edge(a - 1, b - 1);
    }

    R.dfs(X);
    B.dfs(Y);
    vec D = B.d, Par = B.par;
    int ans = 0;
    R.calcans(X, ans, D, Par);
    Ans(ans < INF, ans, -1);

    return 0;
}


Measured memory_bytes: 45044