#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<pair<long long, long long>> v;
	rep(i, n){
		long long l, r;
		cin >> l >> r;
		v.emplace_back(l, r);
	}
	sort(v.begin(), v.end());
	vvi dp(20, vi(n + 1, n));
	rep(i, n){
		auto [l, r] = v[i];
		int idx = lower_bound(v.begin(), v.end(), make_pair(r, (long long)0)) - v.begin();
		dp[0][i] = idx;
	}
	rep(j, 19){
		rep(i, n + 1){
			dp[j + 1][i] = dp[j][dp[j][i]];
		}
	}
	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";
		ans = min(ans, v[p].second - v[i].first);
	}
	if(ans == INF) cout << "-1\n";
	else cout << ans << "\n";
	return 0;
}