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.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.StringTokenizer;

public class E {

	static ArrayList<Integer>[] adjList;
	static int newRoot;

	static int dfs(int u, int p)
	{
		int len = 0;
		for(int v: adjList[u])
			if(v != p)
			{
				int vLen = dfs(v, u);
				if(vLen == -1)
					return -1;
				if(len == 0 || vLen == len)
					len = vLen;
				else
				{
					newRoot = u;
					return -1;
				}
			}
		return len + 1;
	}

	static int dfsRoot(int u)
	{
		int len1 = 0, len2 = 0;
		for(int v: adjList[u])
		{
			int vLen = dfs(v, u);
			if(vLen == -1)
				return -1;
			if(len1 == 0 || vLen == len1)
				len1 = vLen;
			else if(len2 == 0 || vLen == len2)
				len2 = vLen;
			else
				return -1;
		}
		return len1 + len2 + 1;
	}
	public static void sol() throws IOException {
		Scanner sc = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);

		int n = sc.nextInt();
		adjList = new ArrayList[n];
		for(int i = 0; i < n; ++i)
			adjList[i] = new ArrayList<>(1);
		for(int i = 1; i < n; ++i)
		{
			int u = sc.nextInt() - 1, v = sc.nextInt() - 1;
			adjList[u].add(v);
			adjList[v].add(u);
		}
		int ans = dfsRoot(0);
		if(ans == -1)
			ans = dfsRoot(newRoot);
		if(ans != -1)
			--ans;
		out.println(ans);
		out.flush();
		out.close();
	}
	
	public static void main(String[] args) {new Thread(null, new Runnable() { public void run() {try {
		sol();
	} catch (IOException e) {
		
		e.printStackTrace();
	}}}, "2",1<<26).start();}

	static class Scanner 
	{
		StringTokenizer st;
		BufferedReader br;

		public Scanner(InputStream s){	br = new BufferedReader(new InputStreamReader(s));}

		public String next() throws IOException 
		{
			while (st == null || !st.hasMoreTokens()) 
				st = new StringTokenizer(br.readLine());
			return st.nextToken();
		}

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

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

		public String nextLine() throws IOException {return br.readLine();}

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

		public boolean ready() throws IOException {return br.ready();} 
	}
} 