/*
Author: QAQ-Automaton
LANG: C++
PROG: e.cpp
Mail: cnyalilk@vip.qq.com
*/
#include<bits/stdc++.h>
#define debug(...) fprintf(stderr,__VA_ARGS__)
#define DEBUG printf("Passing [%s] in LINE %d\n",__FUNCTION__,__LINE__)
#define Debug debug("Passing [%s] in LINE %d\n",__FUNCTION__,__LINE__) #define all(x) x.begin(),x.end()
#define x first
#define y second
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
const signed inf=0x3f3f3f3f;
const double eps=1e-8;
const double pi=acos(-1.0);
template<class T>int chkmin(T &a,T b){return a>b?a=b,1:0;}
template<class T>int chkmax(T &a,T b){return a<b?a=b,1:0;}
template<class T>T sqr(T a){return a*a;}
template<class T>T mmin(T a,T b){return a<b?a:b;}
template<class T>T mmax(T a,T b){return a>b?a:b;}
template<class T>T aabs(T a){return a<0?-a:a;}
template<class T>int dcmp(T a,T b){return a>b;}
template<int *a>int cmp_a(int x,int y){return a[x]<a[y];}
#define min mmin
#define max mmax
#define abs aabs
namespace io {
	const int SIZE = (1 << 21) + 1;
	char ibuf[SIZE], *iS, *iT, obuf[SIZE], *oS = obuf, *oT = oS + SIZE - 1, c, qu[55]; int f, qr;
	// getchar
	#define gc() (iS == iT ? (iT = (iS = ibuf) + fread (ibuf, 1, SIZE, stdin), (iS == iT ? EOF : *iS ++)) : *iS ++)
	// print the remaining part
	inline void flush () {
		fwrite (obuf, 1, oS - obuf, stdout);
		oS = obuf;
	}
	// putchar
	inline void putc (char x) {
		*oS ++ = x;
		if (oS == oT) flush ();
	}
	// input a signed integer
	inline void read (signed &x) {
		for (f = 1, c = gc(); c < '0' || c > '9'; c = gc()) if (c == '-') f = -1;
		for (x = 0; c <= '9' && c >= '0'; c = gc()) x = x * 10 + (c & 15); x *= f;
	}

	inline void read (long long &x) {
		for (f = 1, c = gc(); c < '0' || c > '9'; c = gc()) if (c == '-') f = -1;
		for (x = 0; c <= '9' && c >= '0'; c = gc()) x = x * 10 + (c & 15); x *= f;
	}
	inline void read (char &x) {
		x=gc();
	}
	inline void read(char *x){
		while((*x=gc())=='\n' || *x==' '||*x=='\r');
		while(!(*x=='\n'||*x==' '||*x=='\r'))*(++x)=gc();
		*x=0;
	}
	template<typename A,typename ...B>
	inline void read(A &x,B &...y){
		read(x);read(y...);
	}
	// print a signed integer
	inline void write (signed x) {
		if (!x) putc ('0'); if (x < 0) putc ('-'), x = -x;
		while (x) qu[++ qr] = x % 10 + '0',  x /= 10;
		while (qr) putc (qu[qr --]);
	}

	inline void write (long long x) {
		if (!x) putc ('0'); if (x < 0) putc ('-'), x = -x;
		while (x) qu[++ qr] = x % 10 + '0',  x /= 10;
		while (qr) putc (qu[qr --]);
	}
	inline void write (char x) {
		putc(x);
	}
	inline void write(const char *x){
		while(*x){putc(*x);++x;}
	}
	inline void write(char *x){
		while(*x){putc(*x);++x;}
	}
	template<typename A,typename ...B>
	inline void write(A x,B ...y){
		write(x);write(y...);
	}
	//no need to call flush at the end manually!
	struct Flusher_ {~Flusher_(){flush();}}io_flusher_;
}
using io :: read;
using io :: putc;
using io :: write;
int n,x,y;
vector<int> R[200005],B[200005];
int loop[200005],dfn[200005],low[200005],t,dis[200005],fa[200005],Dis[200005],Fa[200005];
void BlueDFS(int x,int f){
	fa[x]=f;
	dfn[x]=++t;	
	dis[x]=dis[f]+1;
	for(auto i:B[x])if(i!=f)BlueDFS(i,x);
	low[x]=++t;
}
int chk(int x,int y){
	if(dfn[x]>dfn[y])swap(x,y);
	if(dfn[x]<=dfn[y] && low[y]<=low[x])return dis[y]-dis[x]>2;
	return fa[x]!=fa[y];	
}
int q[200005],*l,*r;
int RedBFS(int x){
	*(l=r=q)=x;	
	int ans=dis[x]<<1;
	if(loop[x])return -1;
	while(l<=r){
		for(auto i:R[*l])if(i!=Fa[*l]){
			Fa[i]=*l;
			Dis[i]=Dis[*l]+1;
			if(Dis[i]>dis[i])continue;
			else chkmax(ans,dis[i]<<1);
			if(Dis[i]<dis[i]){
				*(++r)=i;
				if(loop[i])return -1;
			}
		}
		++l;
	}
	return ans;
}
int main(){
#ifdef QAQAutoMaton 
	freopen("e.in","r",stdin);
	freopen("e.out","w",stdout);
#endif
	read(n,x,y);	
	if(x==y){write("0\n");return 0;}
	int u,v;
	for(int i=1;i<n;++i){read(u,v);R[u].push_back(v);R[v].push_back(u);}
	for(int i=1;i<n;++i){read(u,v);B[u].push_back(v);B[v].push_back(u);}
	dis[0]=-1;
	BlueDFS(y,0);
	for(int i=1;i<=n;++i){
		for(auto j:R[i]){
			if(chk(i,j)){loop[i]=1;break;}
		}
	}
	write(RedBFS(x),'\n');
	return 0;
}



Measured memory_bytes: 40572