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

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

/* constant */

const int MAX_N = 200000;
const int MAX_M = MAX_N * 2;
const int BN = 19;
const int INF = 1 << 30;

/* typedef */

using vi = vector<int>;

/* global variables */

int ls[MAX_N], rs[MAX_N], uxs[MAX_M];
vi nbrs[MAX_M];
int dp[MAX_M], ps[MAX_M][BN];

/* subroutines */

/* main */

int main() {
  int n, k;
  scanf("%d%d", &n, &k);

  int m = 0;
  for (int i = 0; i < n; i++) {
    scanf("%d%d", ls + i, rs + i);
    uxs[m++] = ls[i], uxs[m++] = rs[i];
  }

  sort(uxs, uxs + m);
  m = unique(uxs, uxs + m) - uxs;

  for (int i = 0; i < n; i++) {
    int li = lower_bound(uxs, uxs + m, ls[i]) - uxs;
    int ri = lower_bound(uxs, uxs + m, rs[i]) - uxs;
    nbrs[li].push_back(ri);
  }

  fill(dp, dp + m, -1);
  dp[0] = 0, ps[0][0] = -1;

  for (int i = 0; i < m; i++) {
    if (i + 1 < m) {
      if (dp[i + 1] < dp[i] ||
	  (dp[i + 1] == dp[m] && ps[i + 1][0] < ps[i][0]))
	dp[i + 1] = dp[i], ps[i + 1][0] = ps[i][0];
    }

    for (auto j: nbrs[i]) {
      if (dp[j] < dp[i] + 1 ||
	  (dp[j] == dp[i] + 1 && ps[j][0] < ps[i][0]))
	dp[j] = dp[i] + 1, ps[j][0] = i;
    }
  }
  //for (int i = 0; i < m; i++)
  //printf(" %d,%d,%d", uxs[i], dp[i], ps[i][0]);
  //putchar('\n');

  if (dp[m - 1] < k) { puts("-1"); return 0; }
  
  for (int i = 0; i < BN - 1; i++)
    for (int u = 0; u < m; u++)
      ps[u][i + 1] = (ps[u][i] >= 0) ? ps[ps[u][i]][i] : -1;

  int minl = INF;
  for (int u = m - 1; u >= 0 && dp[u] >= k; u--) {
    int v = u;
    for (int i = 0, bi = 1; i < BN; i++, bi <<= 1)
      if (k & bi) v = ps[v][i];
    minl = min(minl, uxs[u] - uxs[v]);
  }

  printf("%d\n", minl);
  
  return 0;
}