#include <iostream>
#include <string>
#include <cmath>
#include<algorithm>
#include<stack>
#include<queue>
#include<map>
#include<set>
#include<iomanip>
#define _USE_MATH_DEFINES
#include <math.h>
#include <functional>
#include<complex>
#include <cassert>
using namespace std;

#define rep(i,x) for(ll i=0;i<x;i++)
#define repn(i,x) for(ll i=1;i<=x;i++)

typedef long long ll;
const ll INF = 1e17;
const ll MOD = 998244353;
const ll MAX = 4000001;
const double eps = 1E-18;

ll max(ll a, ll b) {
	if (a > b) { return a; }
	return b;
}

ll min(ll a, ll b) {
	if (a > b) { return b; }
	return a;
}

ll gcd(ll a, ll b) {
	if (b == 0) { return a; }
	if (a < b) { return gcd(b, a); }
	return gcd(b, a % b);
}

ll lcm(ll a, ll b) {
	return a / gcd(a, b) * b;
}

struct edge {
	ll ind;
	ll fr;
	ll to;
	ll d;
};

class mint {
	long long x;
public:
	mint(long long x = 0) : x((x% MOD + MOD) % MOD) {}
	mint operator-() const {
		return mint(-x);
	}
	mint& operator+=(const mint& a) {
		if ((x += a.x) >= MOD) x -= MOD;
		return *this;
	}
	mint& operator-=(const mint& a) {
		if ((x += MOD - a.x) >= MOD) x -= MOD;
		return *this;
	}
	mint& operator*=(const  mint& a) {
		(x *= a.x) %= MOD;
		return *this;
	}
	mint operator+(const mint& a) const {
		mint res(*this);
		return res += a;
	}
	mint operator-(const mint& a) const {
		mint res(*this);
		return res -= a;
	}
	mint operator*(const mint& a) const {
		mint res(*this);
		return res *= a;
	}

	mint pow(ll t) const {
		if (!t) return 1;
		mint a = pow(t >> 1);
		a *= a;
		if (t & 1) a *= *this;
		return a;
	}

	// for prime MOD
	mint inv() const {
		return pow(MOD - 2);
	}
	mint& operator/=(const mint& a) {
		return (*this) *= a.inv();
	}
	mint operator/(const mint& a) const {
		mint res(*this);
		return res /= a;
	}

	friend ostream& operator<<(ostream& os, const mint& m) {
		os << m.x;
		return os;
	}
};

mint pw(mint a, ll b) {
	if (b == 0) { return 1; }
	mint ret = pw(a, b >> 1);
	ret *= ret;
	if (b & 1) { ret *= a; }
	return ret;
}

typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;

typedef vector<mint> vmint;
typedef vector<vector<mint>> vvmint;
typedef vector<vector<vector<mint>>> vvvmint;

//////////////////////////////////////
ll N, X, Y;

vvll g;
vvll h;

vll par;
vll ht;

void dfsh(ll u, ll v) {
	par[v] = u;
	ht[v] = ht[u] + 1;
	
	for (ll w : h[v]) {
		if (u == w) { continue; }
		dfsh(v, w);
	}
	return;
}


int main(){
	
	cin >> N >> X >> Y;
	g.resize(N + 1);
	h.resize(N + 1);
	repn(i, N - 1) {
		ll a, b;
		cin >> a >> b;
		g[a].push_back(b);
		g[b].push_back(a);
	}
	repn(i, N - 1) {
		ll c, d;
		cin >> c >> d;
		h[c].push_back(d);
		h[d].push_back(c);
	}

	par.assign(N + 1, 0);
	ht.assign(N + 1, -1);
	dfsh(0, Y);
	//repn(v, N)cout << par[v] << ht[v] << endl;


	vll win(N + 1, 0);
	repn(v, N) {
		for (ll w : g[v]) {
			bool b = 1;
			if (par[w] == v || par[v] == w) { b = 0; }
			if (par[par[w]] == v || par[v] == par[w] || par[par[v]] == w) { b = 0; }
			if (b) { win[v] = 1; break; }
		}

		//cout << v << win[v] << endl;
	}

	set<pair<ll, ll>> st;
	vll lis;
	vll vis(N + 1, 0);

	st.insert({ ht[X],X });
	vis[X] = 1;
	lis.push_back(X);


	ll tern = 0;
	while (!st.empty()) {
		vll nlis;

		vll winlis;

		for (ll v : lis) {
			if (!st.count({ ht[v],v })) { continue; }

			for (ll w : g[v]) {
				if (vis[w] == 1) { continue; }				
				if (ht[w] == tern) { continue; }

				nlis.push_back(w);
				st.insert({ ht[w],w });
				vis[w] = 1;

				if (win[w] == 1) { winlis.push_back(w); }
			}
		}

		lis = nlis;
		while (!st.empty()&&(*st.begin()).first <= tern + 1) {
			st.erase(st.begin());
		}

		for (ll w : winlis) {
			if (st.count({ ht[w], w })) { cout << -1 << endl; return 0; }
		}
		
		tern++;
	
	}

	cout << tern * 2 << endl;
}



Measured memory_bytes: 49700