#include<bits/stdc++.h>
#include<atcoder/all>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef vector<int> vi;
typedef vector<long long> vl;
typedef vector<vector<int>> vvi;
typedef vector<vector<long long>> vvl;
typedef long double ld;
typedef pair<int, int> P;

ostream& operator<<(ostream& os, const modint& a) {os << a.val(); return os;}
template <int m> ostream& operator<<(ostream& os, const static_modint<m>& a) {os << a.val(); return os;}
template<typename T> istream& operator>>(istream& is, vector<T>& v){int n = v.size(); assert(n > 0); rep(i, n) is >> v[i]; return is;}
template<typename U, typename T> ostream& operator<<(ostream& os, const pair<U, T>& p){os << p.first << ' ' << p.second; return os;}
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : " "); return os;}
template <typename T> ostream& operator<<(ostream& os, const vector<vector<T>>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : ""); return os;}

const long long INF = 1001001001001;

int main(){
	int n, k;
	cin >> n >> k;
	vector<tuple<long long, long long, int>> v;
	rep(i, n){
		long long l, r;
		cin >> l >> r;
		v.emplace_back(l, r, -1);
	}
	sort(v.begin(), v.end());
	rep(i, n){
		auto [l, r, tmp] = v[i];
		v[i] = {r, l, i};
	}
	sort(v.begin(), v.end());
	vvi dp(20, vi(n + 1, n));
	queue<pair<long long, int>> qu;
	rep(i, n){
		auto [r, l, id1] = v[i];
//		cout << r << ' ' << l << ' ' << id1 << "\n";
		while(qu.size()){
			auto [r_before, id2] = qu.front();
			if(r_before <= l){
				dp[0][id2] = id1;
				qu.pop(); 
			}else break;
		}
		qu.emplace(r, id1);
	}
	rep(j, 19){
		rep(i, n + 1){
			dp[j + 1][i] = dp[j][dp[j][i]];
		}
	}
	rep(i, n){
		auto [r, l, id] = v[i];
		v[i] = {l, r, id};
	}
	sort(v.begin(), v.end());
	long long ans = INF;
	rep(i, n){
		int p = i;
		rep(j, 20){
			if(((k - 1) >> j) & 1){
				p = dp[j][p];
			}
		}
		if(p == n) continue;
//		cout << i << ' ' << p <<"\n";
		auto [l1, r1, id1] = v[i];
		auto [l2, r2, id2] = v[p];
		ans = min(ans, r2 - l1);
	}
//	cout << dp[0];
	if(ans == INF) cout << "-1\n";
	else cout << ans << "\n";
	return 0;
}