#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic warning "-Wunused-function"
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, k;
  cin >> n >> k;
  vector<P> lr(n);
  for (auto& [l, r] : lr) cin >> l >> r;
  sort(all(lr), [](auto x, auto y) { return x.second < y.second; });
  vector<array<int, 20>> to(n + 1);
  to[n][0] = n;
  struct S {
    int l, r, i;
    bool operator<(const S& rhs) const {
      return l < rhs.l;
    }
  };
  priority_queue<S> q;
  constexpr int INF = 1001001001;
  int rmn = INF, imn = n;
  rrep(i, n) {
    while (q.size() && q.top().l >= lr[i].second) {
      if (q.top().r < rmn) {
        rmn = q.top().r;
        imn = q.top().i;
      }
      q.pop();
    }
    to[i][0] = imn;
    q.push(S{lr[i].first, lr[i].second, i});
  }
  rep(k, 19) rep(i, n + 1) to[i][k+1] = to[to[i][k]][k];
  int ans = INF;
  k--;
  rep(i, n) {
    int v = i;
    rep(j, 20) if (k >> j & 1) v = to[v][j];
    if (v != n) chmin(ans, lr[v].second - lr[i].first);
  }
  if (ans == INF) ans = -1;
  cout << ans << '\n';
}