// package agc.agc005;

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;

public class Main {
    private static final int INF = 100000000;

    public static void main(String[] args) {
        InputReader in = new InputReader(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int n = in.nextInt();
        int A = in.nextInt()-1;
        int B = in.nextInt()-1;
        graphA = buildGraph(in, n, n-1);
        graphB = buildGraph(in, n, n-1);
        distFromA = bfs(graphA, A);
        distFromB = bfs(graphB, B);
        visited = new boolean[n];

        LCA lca = new LCA(graphB);
        canWin = new boolean[n];
        for (int i = 0; i < n ; i++) {
            if (canWin[i]) {
                continue;
            }
            for (int j : graphA[i]) {
                if (lca.dist(i, j) >= 3) {
                    canWin[i] = true;
                    canWin[j] = true;
                    break;
                }
            }
        }

        dfs(A);

        out.println(best >= INF ? -1 : best);
        out.flush();
    }

    static int best = 0;
    static boolean[] canWin;
    static boolean[] visited;

    static void dfs(int now) {
        if (canWin[now]) {
            best = INF;
        }
        visited[now] = true;
        best = Math.max(best, distFromB[now] * 2);
        for (int to : graphA[now]) {
            if (!visited[to] && distFromA[to] < distFromB[to]) {
                dfs(to);
            }
        }
    }

    public static class LCA {
        int[][] graph;
        int[][] parent;
        int[] depth;

        public LCA(int[][] graph) {
            int n = graph.length;
            this.graph = graph;
            init(n);
        }

        void dfs(int now, int from, int dep) {
            parent[0][now] = from;
            depth[now] = dep;
            for (int to : graph[now]) {
                if (to != from) {
                    dfs(to, now, dep+1);
                }
            }
        }

        void init(int n) {
            int log = 1;
            int nn = n;
            while (nn >= 1) {
                nn /= 2;
                log++;
            }
            parent = new int[log+1][n];
            for (int i = 0 ; i <= log ; i++) {
                Arrays.fill(parent[i], -1);
            }
            depth = new int[n];

            dfs(0, -1, 0);

            for (int k = 0 ; k < log ; k++) {
                for (int v = 0 ; v < n ; v++) {
                    if (parent[k][v] < 0) {
                        parent[k+1][v] = -1;
                    } else {
                        parent[k+1][v] = parent[k][parent[k][v]];
                    }
                }
            }
        }

        int lca(int u, int v) {
            int loglen = parent.length;
            if (depth[u] > depth[v]) {
                int tmp = u;
                u = v;
                v = tmp;
            }
            for (int k = 0 ; k < loglen ; k++) {
                if (((depth[v] - depth[u]) >> k) % 2 == 1) {
                    v = parent[k][v];
                }
            }
            if (u == v) {
                return u;
            }

            for (int k = loglen-1 ; k >= 0 ; k--) {
                if (parent[k][u] != parent[k][v]) {
                    u = parent[k][u];
                    v = parent[k][v];
                }
            }
            return parent[0][u];
        }

        int dist(int x, int y) {
            int l = lca(x, y);
            return depth[x] + depth[y] - depth[l] * 2;
        }
    }

    static int[] bfs(int[][] graph, int s) {
        int n = graph.length;
        int[] que = new int[n];
        int[] d = new int[n];
        Arrays.fill(d, INF);


        int qh = 0;
        int qt = 0;
        que[qh++] = s;
        d[s] = 0;
        while (qt < qh) {
            int now = que[qt++];
            int tt = d[now]+1;
            for (int to : graph[now]) {
                if (d[to] > tt) {
                    d[to] = tt;
                    que[qh++] = to;
                }
            }
        }
        return d;
    }

    static int[] distFromA;
    static int[] distFromB;

    static int[][] graphA;
    static int[][] graphB;

    static int[][] buildGraph(InputReader in, int n, int m) {
        int[][] edges = new int[m][];
        int[][] graph = new int[n][];
        int[] deg = new int[n];
        for (int i = 0 ; i < m ; i++) {
            int a = in.nextInt()-1;
            int b = in.nextInt()-1;
            deg[a]++;
            deg[b]++;
            edges[i] = new int[]{a, b};
        }
        for (int i = 0 ; i < n ; i++) {
            graph[i] = new int[deg[i]];
        }
        for (int i = 0 ; i < m ; i++) {
            int a = edges[i][0];
            int b = edges[i][1];
            graph[a][--deg[a]] = b;
            graph[b][--deg[b]] = a;
        }
        return graph;
    }

    public static void debug(Object... o) {
        System.err.println(Arrays.deepToString(o));
    }

    public static class InputReader {
        private static final int BUFFER_LENGTH = 1 << 12;
        private InputStream stream;
        private byte[] buf = new byte[BUFFER_LENGTH];
        private int curChar;
        private int numChars;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        private int next() {
            if (numChars == -1) {
                throw new InputMismatchException();
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public char nextChar() {
            return (char) skipWhileSpace();
        }

        public String nextToken() {
            int c = skipWhileSpace();
            StringBuilder res = new StringBuilder();
            do {
                res.append((char) c);
                c = next();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        public int nextInt() {
            return (int) nextLong();
        }

        public long nextLong() {
            int c = skipWhileSpace();
            long sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = next();
            }
            long res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = next();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public double nextDouble() {
            return Double.valueOf(nextToken());
        }

        int skipWhileSpace() {
            int c = next();
            while (isSpaceChar(c)) {
                c = next();
            }
            return c;
        }

        boolean isSpaceChar(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }
    }
}

Measured memory_bytes: 74168