/* -*- coding: utf-8 -*-
 *
 * 2543.cc:  No.2543 Many Meetings - yukicoder
 */

#include<cstdio>
#include<algorithm>
#include<utility>
 
using namespace std;

/* constant */

const int MAX_N = 200000;
const int BN = 18;
const int INF = 1 << 30;

/* typedef */

using pii = pair<int,int>;

/* global variables */

int ls[MAX_N], rs[MAX_N], ps[MAX_N][BN];
pii rls[MAX_N], lis[MAX_N];

/* subroutines */

/* main */

int main() {
  int n, k;
  scanf("%d%d", &n, &k);
  for (int i = 0; i < n; i++) scanf("%d%d", ls + i, rs + i);

  for (int i = 0; i < n; i++) rls[i] = {rs[i], ls[i]};
  sort(rls, rls + n);

  int c = 0, m = 0;
  for (int i = 0, pr = 0; i < n; i++) {
    auto [ri, li] = rls[i];
    if (pr <= li) c++, pr = ri;
    if (m == 0 || lis[m - 1].first < li) lis[m++] = {li, i};
  }
  if (c < k) { puts("-1"); return 0; }
  
  for (int i = 0; i < n; i++) {
    auto [ri, li] = rls[i];
    int j = lower_bound(lis, lis + m, pii(ri, -1)) - lis;
    ps[i][0] = (j < m) ? lis[j].second : -1;
  }

  for (int i = 0; i < BN - 1; i++)
    for (int u = 0; u < n; u++)
      ps[u][i + 1] = (ps[u][i] >= 0) ? ps[ps[u][i]][i] : -1;

  int minl = INF;
  k--;
  for (int u = 0; u < n; u++) {
    int v = u;
    for (int i = 0, bi = 1; v >= 0 && i < BN; i++, bi <<= 1)
      if (k & bi) v = ps[v][i];
    if (v < 0) break;

    minl = min(minl, rls[v].first - rls[u].second);
  }

  printf("%d\n", (minl < INF) ? minl : -1);
  
  return 0;
}