import java.io.PrintWriter;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Scanner;
import java.math.*;

class Main {
	public static void main(String[] args) throws Exception {
		new Main().run();
	}
	
	void run() {
		Scanner sc=new Scanner(System.in);
		int N=sc.nextInt();
		int K=sc.nextInt();
		int[] a=new int[N-1];
		int[] b=new int[N-1];
		for(int i=0;i<N-1;++i){
			a[i]=sc.nextInt();
			b[i]=sc.nextInt();
		}
		System.out.println(N>=K?K-1:-1);
	}

	static void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}

}