import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.util.Queue;

public class Main {
	static InputStream is;
	static PrintWriter out;
	static String INPUT = "";
	
	static void solve()
	{
		int n = ni(), x = ni()-1, y = ni()-1;
		int[][] gx = ng(n);
		int[][] gy = ng(n);
		int[][] psx = parents3(gx, x);
		int[][] psy = parents3(gy, y);
		int[] parx = psx[0], depx = psx[2];
		int[] pary = psy[0], depy = psy[2];
		int[][] spary = logstepParents(pary);
		boolean[] safe = new boolean[n];
		// Aで距離1, Bで距離3以上なら両端はセーフ
		for(int i = 0;i < n;i++){
			if(i != x){
				int la = parx[i], lb = i;
				int lca = lca2(la, lb, spary, depy);
				if((depx[la] < depy[la] || depx[lb] < depy[lb]) && depy[la] + depy[lb] - 2*depy[lca] >= 3){
					safe[parx[i]] = true;
					safe[i] = true;
				}
			}
		}
		// dA < dBとなっている場所を通っていける場所
		Queue<Integer> q = new ArrayDeque<>();
		q.add(x);
		boolean[] ved = new boolean[n];
		ved[x] = true;
		while(!q.isEmpty()){
			int cur = q.poll();
			if(depx[cur] < depy[cur]){
				for(int e : gx[cur]){
					if(!ved[e] && depx[e] <= depy[e]){
						ved[e] = true;
						q.add(e);
					}
				}
			}
		}
		int maxdy = 0;
		for(int i = 0;i < n;i++){
			if(ved[i]){
				maxdy = Math.max(maxdy, depy[i]);
				if(safe[i]){
					out.println(-1);
					return;
				}
			}
		}
		out.println(maxdy*2);
	}
	
	public static int lca2(int a, int b, int[][] spar, int[] depth) {
		if (depth[a] < depth[b]) {
			b = ancestor(b, depth[b] - depth[a], spar);
		} else if (depth[a] > depth[b]) {
			a = ancestor(a, depth[a] - depth[b], spar);
		}

		if (a == b)
			return a;
		int sa = a, sb = b;
		for (int low = 0, high = depth[a], t = Integer.highestOneBit(high), k = Integer
				.numberOfTrailingZeros(t); t > 0; t >>>= 1, k--) {
			if ((low ^ high) >= t) {
				if (spar[k][sa] != spar[k][sb]) {
					low |= t;
					sa = spar[k][sa];
					sb = spar[k][sb];
				} else {
					high = low | t - 1;
				}
			}
		}
		return spar[0][sa];
	}

	protected static int ancestor(int a, int m, int[][] spar) {
		for (int i = 0; m > 0 && a != -1; m >>>= 1, i++) {
			if ((m & 1) == 1)
				a = spar[i][a];
		}
		return a;
	}

	public static int[][] logstepParents(int[] par) {
		int n = par.length;
		int m = Integer.numberOfTrailingZeros(Integer.highestOneBit(n - 1)) + 1;
		int[][] pars = new int[m][n];
		pars[0] = par;
		for (int j = 1; j < m; j++) {
			for (int i = 0; i < n; i++) {
				pars[j][i] = pars[j - 1][i] == -1 ? -1 : pars[j - 1][pars[j - 1][i]];
			}
		}
		return pars;
	}

	
	public static int[][] parents3(int[][] g, int root) {
		int n = g.length;
		int[] par = new int[n];
		Arrays.fill(par, -1);

		int[] depth = new int[n];
		depth[0] = 0;

		int[] q = new int[n];
		q[0] = root;
		for (int p = 0, r = 1; p < r; p++) {
			int cur = q[p];
			for (int nex : g[cur]) {
				if (par[cur] != nex) {
					q[r++] = nex;
					par[nex] = cur;
					depth[nex] = depth[cur] + 1;
				}
			}
		}
		return new int[][] { par, q, depth };
	}

	
	static int[][] ng(int n)
	{
		int[] from = new int[n-1];
		int[] to = new int[n-1];
		for(int i = 0;i < n-1;i++){
			from[i] = ni()-1;
			to[i] = ni()-1;
		}
		int[][] g = packU(n, from, to);
		return g;
	}
	
	static int[][] packU(int n, int[] from, int[] to) {
		int[][] g = new int[n][];
		int[] p = new int[n];
		for (int f : from)
			p[f]++;
		for (int t : to)
			p[t]++;
		for (int i = 0; i < n; i++)
			g[i] = new int[p[i]];
		for (int i = 0; i < from.length; i++) {
			g[from[i]][--p[from[i]]] = to[i];
			g[to[i]][--p[to[i]]] = from[i];
		}
		return g;
	}
	
	public static void main(String[] args) throws Exception
	{
		long S = System.currentTimeMillis();
		is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes());
		out = new PrintWriter(System.out);
		
		solve();
		out.flush();
		long G = System.currentTimeMillis();
		tr(G-S+"ms");
	}
	
	private static boolean eof()
	{
		if(lenbuf == -1)return true;
		int lptr = ptrbuf;
		while(lptr < lenbuf)if(!isSpaceChar(inbuf[lptr++]))return false;
		
		try {
			is.mark(1000);
			while(true){
				int b = is.read();
				if(b == -1){
					is.reset();
					return true;
				}else if(!isSpaceChar(b)){
					is.reset();
					return false;
				}
			}
		} catch (IOException e) {
			return true;
		}
	}
	
	private static byte[] inbuf = new byte[1024];
	static int lenbuf = 0, ptrbuf = 0;
	
	private static int readByte()
	{
		if(lenbuf == -1)throw new InputMismatchException();
		if(ptrbuf >= lenbuf){
			ptrbuf = 0;
			try { lenbuf = is.read(inbuf); } catch (IOException e) { throw new InputMismatchException(); }
			if(lenbuf <= 0)return -1;
		}
		return inbuf[ptrbuf++];
	}
	
	private static boolean isSpaceChar(int c) { return !(c >= 33 && c <= 126); }
//	private static boolean isSpaceChar(int c) { return !(c >= 32 && c <= 126); }
	private static int skip() { int b; while((b = readByte()) != -1 && isSpaceChar(b)); return b; }
	
	private static double nd() { return Double.parseDouble(ns()); }
	private static char nc() { return (char)skip(); }
	
	private static String ns()
	{
		int b = skip();
		StringBuilder sb = new StringBuilder();
		while(!(isSpaceChar(b))){
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}
	
	private static char[] ns(int n)
	{
		char[] buf = new char[n];
		int b = skip(), p = 0;
		while(p < n && !(isSpaceChar(b))){
			buf[p++] = (char)b;
			b = readByte();
		}
		return n == p ? buf : Arrays.copyOf(buf, p);
	}
	
	private static char[][] nm(int n, int m)
	{
		char[][] map = new char[n][];
		for(int i = 0;i < n;i++)map[i] = ns(m);
		return map;
	}
	
	private static int[] na(int n)
	{
		int[] a = new int[n];
		for(int i = 0;i < n;i++)a[i] = ni();
		return a;
	}
	
	private static int ni()
	{
		int num = 0, b;
		boolean minus = false;
		while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
		if(b == '-'){
			minus = true;
			b = readByte();
		}
		
		while(true){
			if(b >= '0' && b <= '9'){
				num = num * 10 + (b - '0');
			}else{
				return minus ? -num : num;
			}
			b = readByte();
		}
	}
	
	private static long nl()
	{
		long num = 0;
		int b;
		boolean minus = false;
		while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
		if(b == '-'){
			minus = true;
			b = readByte();
		}
		
		while(true){
			if(b >= '0' && b <= '9'){
				num = num * 10 + (b - '0');
			}else{
				return minus ? -num : num;
			}
			b = readByte();
		}
	}
	
	private static void tr(Object... o) { if(INPUT.length() != 0)System.out.println(Arrays.deepToString(o)); }
}


Measured memory_bytes: 59708