#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  ll N, K;
  cin >> N >> K;

  vector<pair<ll, ll>> v(N);
  for(auto &[x, y] : v) { cin >> x >> y; }
  ranges::sort(v, {}, &pair<ll, ll>::second);

  set<tuple<ll, ll, ll>> s;
  for(ll i = 0; i < N; i++) {
    auto [l, r] = v[i];
    s.emplace(r, l, i);
  }

  vector<ll> sortidx(N);
  iota(sortidx.begin(), sortidx.end(), 0);
  ranges::sort(sortidx, [&](auto i, auto j) { return v[i] < v[j]; });

  vector<vector<ll>> nxt(20, vector<ll>(N, -1));
  ll idx = 0;
  for(ll i = 0; i < N; i++) {
    auto [l, r] = v[i];
    while(idx < N && v[sortidx[idx]].first < r) {
      s.erase({v[sortidx[idx]].second, v[sortidx[idx]].first, sortidx[idx]});
      idx++;
    }
    if(s.empty()) { break; }
    nxt[0][i] = get<2>(*s.begin());
  }

  for(ll i = 0; i < 19; i++) {
    for(ll j = 0; j < N; j++) {
      if(nxt[i][j] == -1) { continue; }
      nxt[i + 1][j] = nxt[i][nxt[i][j]];
    }
  }

  ll r = 1e18;
  for(ll i = 0; i < N; i++) {
    ll k = K - 1, cur = i;
    while(k && cur != -1) {
      ll t = __builtin_ctz(k);
      cur = nxt[t][cur];
      k ^= 1 << t;
    }
    if(cur != -1) { r = min(r, v[cur].second - v[i].first); }
  }

  cout << (r == 1e18 ? -1 : r) << "\n";
}