#include <bits/stdc++.h>
using namespace std;

using ll=long long;
#define int ll

#define rng(i,a,b) for(int i=int(a);i<int(b);i++)
#define rep(i,b) rng(i,0,b)
#define gnr(i,a,b) for(int i=int(b)-1;i>=int(a);i--)
#define per(i,b) gnr(i,0,b)
#define pb push_back
#define eb emplace_back
#define a first
#define b second
#define bg begin()
#define ed end()
#define all(x) x.bg,x.ed
#ifdef LOCAL
#define dmp(x) cerr<<__LINE__<<" "<<#x<<" "<<x<<endl
#else
#define dmp(x) void(0)
#endif

template<class t,class u> void chmax(t&a,u b){if(a<b)a=b;}
template<class t,class u> void chmin(t&a,u b){if(b<a)a=b;}

template<class t> using vc=vector<t>;
template<class t> using vvc=vc<vc<t>>;

using pi=pair<int,int>;
using vi=vc<int>;

template<class t,class u>
ostream& operator<<(ostream& os,const pair<t,u>& p){
	return os<<"{"<<p.a<<","<<p.b<<"}";
}

template<class t> ostream& operator<<(ostream& os,const vc<t>& v){
	os<<"{";
	for(auto e:v)os<<e<<",";
	return os<<"}";
}

#define mp make_pair
#define mt make_tuple
#define one(x) memset(x,-1,sizeof(x))
#define zero(x) memset(x,0,sizeof(x))
#ifdef LOCAL
void dmpr(ostream&os){os<<endl;}
template<class T,class... Args>
void dmpr(ostream&os,const T&t,const Args&... args){
	os<<t<<" ";
	dmpr(os,args...);
}
#define dmp2(...) dmpr(cerr,"Line:",__LINE__,##__VA_ARGS__)
#else
#define dmp2(...) void(0)
#endif

using uint=unsigned;
using ull=unsigned long long;

template<int i,class T>
void print_tuple(ostream&,const T&){
}

template<int i,class T,class H,class ...Args>
void print_tuple(ostream&os,const T&t){
	if(i)os<<",";
	os<<get<i>(t);
	print_tuple<i+1,T,Args...>(os,t);
}

template<class ...Args>
ostream& operator<<(ostream&os,const tuple<Args...>&t){
	os<<"{";
	print_tuple<0,tuple<Args...>,Args...>(os,t);
	return os<<"}";
}

void print(ll x,int suc=1){
	cout<<x;
	if(suc==1)
		cout<<"\n";
	if(suc==2)
		cout<<" ";
}

ll read(){
	ll i;
	cin>>i;
	return i;
}

vi readvi(int n,int off=0){
	vi v(n);
	rep(i,n)v[i]=read()+off;
	return v;
}

template<class T>
void print(const vector<T>&v,int suc=1){
	rep(i,v.size())
		print(v[i],i==int(v.size())-1?suc:2);
}

string readString(){
	string s;
	cin>>s;
	return s;
}

template<class T>
T sq(const T& t){
	return t*t;
}

//#define CAPITAL
void yes(bool ex=true){
	#ifdef CAPITAL
	cout<<"YES"<<endl;
	#else
	cout<<"Yes"<<endl;
	#endif
	if(ex)exit(0);
}
void no(bool ex=true){
	#ifdef CAPITAL
	cout<<"NO"<<endl;
	#else
	cout<<"No"<<endl;
	#endif
	if(ex)exit(0);
}

constexpr ll ten(int n){
	return n==0?1:ten(n-1)*10;
}

const ll infLL=LLONG_MAX/3;

#ifdef int
const int inf=infLL;
#else
const int inf=INT_MAX/2-100;
#endif

int topbit(signed t){
	return t==0?-1:31-__builtin_clz(t);
}
int topbit(ll t){
	return t==0?-1:63-__builtin_clzll(t);
}
int botbit(signed a){
	return a==0?32:__builtin_ctz(a);
}
int botbit(ll a){
	return a==0?64:__builtin_ctzll(a);
}
int popcount(signed t){
	return __builtin_popcount(t);
}
int popcount(ll t){
	return __builtin_popcountll(t);
}
bool ispow2(int i){
	return i&&(i&-i)==i;
}
int mask(int i){
	return (int(1)<<i)-1;
}

bool inc(int a,int b,int c){
	return a<=b&&b<=c;
}

template<class t> void mkuni(vc<t>&v){
	sort(all(v));
	v.erase(unique(all(v)),v.ed);
}

ll rand_int(ll l, ll r) { //[l, r]
	#ifdef LOCAL
	static mt19937_64 gen;
	#else
    static random_device rd;
    static mt19937_64 gen(rd());
    #endif
    return uniform_int_distribution<ll>(l, r)(gen);
}

//CF530F
//CodeChef Persistent Oak
//AOJ GRL5C
template<class E>
struct HLD{
	vvc<E> g;
	int n,rt,cnt;
	vi sub,in,out,par,head,dep;
	int dfs1(int v,int p,int d){
		par[v]=p;
		dep[v]=d;
		g[v].erase(remove(all(g[v]),p),g[v].ed);
		for(auto&e:g[v]){
			sub[v]+=dfs1(e,v,d+1);
			if(sub[g[v][0]]<sub[e])
				swap(g[v][0],e);
		}
		return sub[v];
	}
	void dfs2(int v,int h){
		in[v]=cnt++;
		head[v]=h;
		for(int to:g[v])
			dfs2(to,to==g[v][0]?h:to);
		out[v]=cnt;
	}
	HLD(const vvc<E>&gg,int rr):g(gg),n(g.size()),rt(rr),cnt(0),
		sub(n,1),in(n),out(n),par(n,-1),head(n),dep(n){
		dfs1(rt,-1,0);
		dfs2(rt,rt);
	}
	int lca(int a,int b){
		while(head[a]!=head[b]){
			if(dep[head[a]]>dep[head[b]])
				swap(a,b);
			b=par[head[b]];
		}
		if(dep[a]>dep[b])
			swap(a,b);
		return a;
	}
	int len(int a,int b){
		return dep[a]+dep[b]-dep[lca(a,b)]*2;
	}
	bool asde(int a,int b){
		return in[a]<=in[b]&&out[b]<=out[a];
	}
};

signed main(){
	cin.tie(0);
	ios::sync_with_stdio(0);
	cout<<fixed<<setprecision(20);
	
	int n,x,y;cin>>n>>x>>y;
	x--;y--;
	vvc<int> g(n);
	rep(_,n-1){
		int a,b;cin>>a>>b;
		a--;b--;
		g[a].pb(b);
		g[b].pb(a);
	}
	vvc<int> t(n);
	rep(_,n-1){
		int a,b;cin>>a>>b;
		a--;b--;
		t[a].pb(b);
		t[b].pb(a);
	}
	
	auto h=HLD<int>(t,y);
	
	vi dist(n,-1);
	queue<int> q;
	auto rc=[&](int v,int d){
		if(dist[v]!=-1)return;
		if(d>=h.dep[v])return;
		dist[v]=d;
		q.push(v);
	};
	
	rc(x,0);
	while(q.size()){
		int v=q.front();q.pop();
		for(auto to:g[v])
			rc(to,dist[v]+1);
	}
	
	int mx=0;
	rep(i,n)if(dist[i]!=-1){
		for(auto j:g[i])
			if(h.len(i,j)>=3){
				cout<<-1<<endl;
				return 0;
			}
		chmax(mx,h.dep[i]);
	}
	
	cout<<mx*2<<endl;
}


Measured memory_bytes: 54776