/* package whatever; // don't place package name! */

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

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
		public static void main (String[] args) throws java.lang.Exception
	{
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int m = sc.nextInt();
		List<List<Integer>> adj = new ArrayList<>();
		for(int i=0; i<=n;i++){
			adj.add(new ArrayList<>());
		}
 
		for(int i=0; i<m; i++){
			int u = sc.nextInt();
			int v = sc.nextInt();
			adj.get(u).add(v);
			adj.get(v).add(u);
		}
		int ans = -1;
		for(int i=1;i<n;i++){
			if(adj.get(i).size()==1){
				ans = i;
			}
		}
		System.out.println(ans);
 
	}
}