#include<iostream>
#include<string>
#include<cstdio>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<utility>
#include<cassert>
#include<complex>
using namespace std;

//#define int long long
typedef long long ll;

typedef unsigned long long ul;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = (1e+18) + 7;
typedef pair<int, int>P;
#define stop char nyaa;cin>>nyaa;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
#define all(v) (v).begin(),(v).end()
typedef pair<ll, ll> LP;
typedef long double ld;
typedef pair<ld, ld> LDP;
const ld eps = 1e-6;
const ld pi = acos(-1.0);
//typedef vector<vector<ll>> mat;
typedef vector<int> vec;

ll mod_pow(ll a, ll n) {
	ll res = 1;
	while (n) {
		if (n & 1)res = res * a%mod;
		a = a * a%mod; n >>= 1;
	}
	return res;
}

struct modint {
	ll n;
	modint() :n(0) { ; }
	modint(ll m) :n(m) {
		if (n >= mod)n %= mod;
		else if (n < 0)n = (n%mod + mod) % mod;
	}
	operator int() { return n; }
};
bool operator==(modint a, modint b) { return a.n == b.n; }
modint operator+=(modint &a, modint b) { a.n += b.n; if (a.n >= mod)a.n -= mod; return a; }
modint operator-=(modint &a, modint b) { a.n -= b.n; if (a.n < 0)a.n += mod; return a; }
modint operator*=(modint &a, modint b) { a.n = ((ll)a.n*b.n) % mod; return a; }
modint operator+(modint a, modint b) { return a += b; }
modint operator-(modint a, modint b) { return a -= b; }
modint operator*(modint a, modint b) { return a *= b; }
modint operator^(modint a, int n) {
	if (n == 0)return modint(1);
	modint res = (a*a) ^ (n / 2);
	if (n % 2)res = res * a;
	return res;
}

//ll inv(ll a, ll p) {
//	return (a == 1 ? 1 : (1 - p * inv(p%a, a)) / a + p);
//}
//modint operator/(modint a, modint b) { return a * modint(inv(b, mod)); }

int dx[4] = { 1,0,-1,0 };
int dy[4] = { 0,1,0,-1 };

vector<int> G[1 << 18];

int depth[1 << 18];
int par[1 << 18][18];

void dfs(int id, int fr) {
	if (fr >= 0)depth[id] = depth[fr] + 1;
	par[id][0] = fr;
	for (int to : G[id]) {
		if (to == fr)continue;
		dfs(to, id);
	}
}
int n;
void yaru(int root) {
	dfs(root, -1);
	rep(j, 17) {
		rep(i, n) {
			if (par[i][j] < 0)par[i][j + 1] = -1;
			else par[i][j + 1] = par[par[i][j]][j];
		}
	}
}
int lca(int a, int b) {
	if (depth[a] < depth[b])swap(a, b);
	rep(k, 18) {
		if ((depth[a] - depth[b]) >> k & 1) {
			a = par[a][k];
		}
	}
	if (a == b)return a;
	per(k, 18) {
		if (par[a][k] != par[b][k]) {
			a = par[a][k], b = par[b][k];
		}
	}
	return par[a][0];
}
int dist(int a, int b) {
	int l = lca(a, b);
	return depth[a] + depth[b] - 2 * depth[l];
}

bool inf[1 << 18];
vector<int> g[1 << 18];

int d[1 << 18];
void solve() {
	cin >> n;
	int x, y; cin >> x >> y; x--; y--;
	vector<P> v(n-1);
	rep(i, n - 1) {
		int a, b; cin >> a >> b; a--; b--;
		v[i] = { a,b };
	}
	rep(i, n-1) {
		int a, b; cin >> a >> b; a--; b--;
		G[a].push_back(b);
		G[b].push_back(a);
	}
	yaru(y);
	rep(i, n - 1) {
		int l = v[i].first, r = v[i].second;
		int d = dist(l,r);
		if (d >= 3) {
			inf[l] =inf[r]= true;
		}
		else {
			g[l].push_back(r);
			g[r].push_back(l);
		}
	}
	fill(d, d + n, mod);
	d[x] = 0;
	queue<int> q; q.push(x);
	while (!q.empty()) {
		int id = q.front(); q.pop();
		for (int to : g[id]) {
			int nd = d[id] + 1;
			if (nd < d[to] && nd < depth[to]) {
				d[to] = nd; q.push(to);
			}
		}
	}
	int ma = 0;
	rep(i, n) {
		if (d[i] == mod)continue;
		if (inf[i]) {
			cout << -1 << endl; return;
		}
		//cout << i << " " << depth[i] << endl;
		ma = max(ma, depth[i]);
	}
	cout << 2 * ma << endl;
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	//cout << fixed << setprecision(7);
	//init();
	//int t; cin >> t; rep(i, t)solve();
	solve();
	stop
		return 0;
}


Measured memory_bytes: 53120