#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
//using mint = modint998244353;
//const int mod = 998244353;
//using mint = modint1000000007;
//const int mod = 1000000007;
const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
P op(P a, P b) {
	if (a.first < b.first)return a;
	if (a.first > b.first)return b;
	if (a.second < b.second)return a;
	return b;
}
P e() { return { INF * 2,INF * 2 }; }
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n, k; cin >> n >> k;
	vector<P>x(n);
	rep(i, n) {
		cin >> x[i].first >> x[i].second;
	}
	x.push_back({ INF * 2,INF * 2 });
	sort(all(x));
	segtree<P, op, e>seg(x.size());
	rep(i, x.size()) {
		seg.set(i, { x[i].second ,i });
	}
	vector<vector<int>>to(n + 1);
	rep(i, n) {
		P p = { x[i].second,x[i].second };
		int idx = upper_bound(all(x), p) - x.begin();
		auto get = seg.prod(idx, n + 1);
		to[get.second].push_back(i);
	}
	vector<P>list;
	int ans = INF * 2;
	auto dfs = [&](auto self, int v, int p = -1)->void {
		if (-1 != p)list.push_back(x[v]);
		if (list.size() >= k) {
			auto l = list.back();
			auto r = list[list.size() - k];
			auto val = r.second - l.first;
			chmin(ans, val);
		}
		for (auto nv : to[v]) {
			if (p == nv)continue;
			self(self, nv, v);
		}
		if (-1 != p)list.pop_back();
	};
	dfs(dfs, n);
	if (INF * 2 == ans)ans = -1;
	cout << ans << endl;
	return 0;
}