10
4 2
7 4
2 6
2 5
4 8
10 3
2 9
9 1
5 10
 10
5 10
7 8
8 3
2 6
3 2
9 7
4 5
10 1
6 4
 4
4 1
4 3
4 2
 3
3 1
1 2
 9
1 2
1 3
1 4
1 5
1 6
6 7
6 8
8 9
 5
1 5
2 3
2 4
1 2
 8
6 1
4 7
4 8
8 5
7 6
4 3
4 2
 7
5 6
5 7
5 1
7 4
6 3
3 2
 3
1 3
3 2
 2
1 2
 5
5 4
4 3
3 1
5 2
 6
1 6
3 1
6 4
5 3
2 5
 11
8 9
2 7
1 11
3 2
9 1
8 5
8 6
5 4
4 10
8 3
 11
11 9
6 7
7 1
8 11
5 6
3 5
9 3
10 8
2 4
4 10
 12
12 6
6 7
8 11
4 8
10 4
12 3
2 10
6 2
12 9
4 1
9 5

-1
 9
 1
 1
 3
 3
 -1
 -1
 1
 1
 1
 5
 1
 5
 -1

import java.io.*;
import java.util.*;

public class E {
    @SuppressWarnings("unchecked")
    public static void solution(BufferedReader reader, PrintWriter writer)
            throws IOException {
        In in = new In(reader);
        Out out = new Out(writer);
        int n = in.nextInt();
        adj = new ArrayList[n + 1];
        for (int i = 1; i <= n; i++)
            adj[i] = new ArrayList<Integer>();
        for (int i = 1; i < n; i++) {
            int u = in.nextInt(), v = in.nextInt();
            adj[u].add(v);
            adj[v].add(u);
        }

        d = new int[n + 1][n + 1];
        for (int i = 1; i <= n; i++)
            Arrays.fill(d[i], Integer.MAX_VALUE);
        int min = Integer.MAX_VALUE;
        for (int r = 1; r <= n; r++) {
            HashMap<Integer, Integer> ds = new HashMap<Integer, Integer>();
            for (int u : adj[r]) {
                int dru = solve(u, r);
                if (!ds.containsKey(dru))
                    ds.put(dru, 0);
                ds.put(dru, ds.get(dru) + 1);
            }
            if (ds.containsKey(-1))
                continue;
            if (ds.size() == 1)
                for (Integer dru : ds.keySet())
                    min = Math.min(min, dru);
            if (ds.size() == 2) {
                int sum = 0;
                for (Integer dru : ds.keySet())
                    sum += dru;
                min = Math.min(min, sum);
            }
        }
        out.println(min == Integer.MAX_VALUE ? -1 : min);
    }

    private static ArrayList<Integer>[] adj;
    private static int[][] d;

    private static int solve(int u, int p) {
        if (d[u][p] != Integer.MAX_VALUE)
            return d[u][p];
        if (adj[u].size() == 1)
            d[u][p] = 1;
        for (int v : adj[u])
            if (v != p) {
                int dvu = solve(v, u);
                if (dvu == -1) {
                    d[u][p] = -1;
                    break;
                }
                if (d[u][p] == Integer.MAX_VALUE)
                    d[u][p] = dvu + 1;
                else if (d[u][p] != dvu + 1) {
                    d[u][p] = -1;
                    break;
                }
            }
        return d[u][p];
    }

    public static void main(String[] args) throws Exception {
        BufferedReader reader = new BufferedReader(
                new InputStreamReader(System.in));
        PrintWriter writer = new PrintWriter(
                new BufferedWriter(new OutputStreamWriter(System.out)));
        solution(reader, writer);
        writer.close();
    }

    protected static class In {
        private BufferedReader reader;
        private StringTokenizer tokenizer = new StringTokenizer("");

        public In(BufferedReader reader) {
            this.reader = reader;
        }

        public String next() throws IOException {
            while (!tokenizer.hasMoreTokens())
                tokenizer = new StringTokenizer(reader.readLine());
            return tokenizer.nextToken();
        }

        public int nextInt() throws IOException {
            return Integer.parseInt(next());
        }

        public long nextLong() throws IOException {
            return Long.parseLong(next());
        }

        public double nextDouble() throws IOException {
            return Double.parseDouble(next());
        }

        public int[] nextIntArray(int n) throws IOException {
            int[] a = new int[n];
            for (int i = 0; i < n; i++)
                a[i] = nextInt();
            return a;
        }

        public int[] nextIntArray1(int n) throws IOException {
            int[] a = new int[n + 1];
            for (int i = 1; i <= n; i++)
                a[i] = nextInt();
            return a;
        }

        public int[] nextIntArraySorted(int n) throws IOException {
            int[] a = nextIntArray(n);
            Random r = new Random();
            for (int i = 0; i < n; i++) {
                int j = i + r.nextInt(n - i);
                int t = a[i];
                a[i] = a[j];
                a[j] = t;
            }
            Arrays.sort(a);
            return a;
        }

        public long[] nextLongArray(int n) throws IOException {
            long[] a = new long[n];
            for (int i = 0; i < n; i++)
                a[i] = nextLong();
            return a;
        }

        public long[] nextLongArray1(int n) throws IOException {
            long[] a = new long[n + 1];
            for (int i = 1; i <= n; i++)
                a[i] = nextLong();
            return a;
        }

        public long[] nextLongArraySorted(int n) throws IOException {
            long[] a = nextLongArray(n);
            Random r = new Random();
            for (int i = 0; i < n; i++) {
                int j = i + r.nextInt(n - i);
                long t = a[i];
                a[i] = a[j];
                a[j] = t;
            }
            Arrays.sort(a);
            return a;
        }
    }

    protected static class Out {
        private PrintWriter writer;
        private static boolean local = System
                .getProperty("ONLINE_JUDGE") == null;

        public Out(PrintWriter writer) {
            this.writer = writer;
        }

        public void print(char c) {
            writer.print(c);
        }

        public void print(int a) {
            writer.print(a);
        }

        public void println(Object a) {
            writer.println(a);
        }

        public void println(Object[] os) {
            for (int i = 0; i < os.length; i++) {
                writer.print(os[i]);
                writer.print(' ');
            }
            writer.println();
        }

        public void println(int[] a) {
            for (int i = 0; i < a.length; i++) {
                writer.print(a[i]);
                writer.print(' ');
            }
            writer.println();
        }

        public void println(long[] a) {
            for (int i = 0; i < a.length; i++) {
                writer.print(a[i]);
                writer.print(' ');
            }
            writer.println();
        }

        public static void db(Object... objects) {
            if (local)
                System.out.println(Arrays.deepToString(objects));
        }
    }
}
