//#include <bits/stdc++.h>
#include <iostream> // cout, endl, cin
#include <string> // string, to_string, stoi
#include <vector> // vector
#include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
#include <utility> // pair, make_pair
#include <tuple> // tuple, make_tuple
#include <cstdint> // int64_t, int*_t
#include <cstdio> // printf
#include <map> // map
#include <queue> // queue, priority_queue
#include <set> // set
#include <stack> // stack
#include <deque> // deque
#include <unordered_map> // unordered_map
#include <unordered_set> // unordered_set
#include <bitset> // bitset
#include <cctype> // isupper, islower, isdigit, toupper, tolower
 
using namespace std;

#define rep(i,n) for(int i = 0; i < (n); i++)
#define pb push_back 
#define eb emplace_back 
#define all(x) x.begin() , x.end()
#define fi frist
#define se second
#define Yes() {puts("YES");return;}
#define No()  {puts("No"); return;}
#define P pair <int , int>
#define vi vector<int> 
#define vvi vector<<vector<int>> 
#define vs vector<string> 

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
 
const ll mod = 1000000007;
//const ll mod = 998244353;
const ld pi = 3.14159265358979;
const int INF = 100100100;

/*
[memo]
v.erase(unique(v.begin(), v.end()), v.end());
*/

int in() { int x;scanf("%d\n" , &x);return x;}
ll lin() { ll x;scanf("%lld\n" , &x);return x;}
ll gcd (ll a , ll b) {if (a < b) swap(a , b);if (b == 0) return 1;if (a % b == 0) return b;else return gcd (b , a % b);}

bool is_prime (ll x) {
	if (x == 1) {
		return false;
	}
	if (x == 2 || x == 3 || x == 5 || x == 7) {
		return true;
	}
	if((x & 1) == 0){
		return false;
	}
	for (ll i = 3; i * i <= x; i += 2) {
		if(x % i == 0)return false;
	}
	return true;
}

bool is_2power(ll x) {
	if((x & (x - 1)) == 0) {return true;}
	else {return false;}
}

vector<int> divisor (ll x) {
	vector<int> ret;
	for (int i = 2; i * i <= x; i++) {
		while (x % i == 0) {
			ret.push_back(i);
			x /= i;
		}
	}
	if (x > 1) {
		ret.push_back(x);
	}
	return ret;
}

ll modpow (ll x , ll p , ll m) {
	ll ret = 1;
	while (p > 0) {
		if(p & 1)ret *= x;
		x *= x;
		x %= m;
		p >>= 1;
	}
	return ret;
}

//vector<string> ans;

void solve() {
	int n , k;
	cin >> n >> k;
	int a , b;
	rep(i,n - 1){
		cin >> a >> b;
	}
	if(k > n){
		cout << -1 << endl;
	}else{
		cout << k - 1 << endl;
 	}
	return; 
}

int main() {	
	cin.tie(0);
	ios::sync_with_stdio(false);
	//cout << fixed << setprecision(15);
	//int t = in();
	int t = 1;
	rep(i,t)solve();
	return 0;
}