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

int main(void) {
  int N; size_t K; scanf("%d%lu", &N, &K);
  vector<int> a(N), b(N);
  vector<int> v(N*2);
  for(int i=0, j=0; i<N; ++i) {
    scanf("%d%d", &a[i], &b[i]);
    v[j++] = a[i];
    v[j++] = b[i] + 1;
  }
  vector<int> w = v;
  sort(w.begin(), w.end()); w.erase(unique(w.begin(), w.end()), w.end());
  vector<size_t> imos(N*2 + 1);
  for(int i=0, j=0; i<N; ++i) {
    size_t l = lower_bound(w.begin(), w.end(), v[j++]) - w.begin(),
           r = lower_bound(w.begin(), w.end(), v[j++]) - w.begin();
    ++imos[l];
    --imos[r];
  }
  for(int i=0; i<N*2; ++i) {
    imos[i+1] += imos[i];
  }
  size_t cnt = 0;
  size_t dif, ncnt;
  for(size_t i=0, n=w.size(); i<n-1; ++i) {
    dif = w[i+1] - w[i];
    ncnt = cnt + dif * imos[i];
    if(ncnt < K) {
      cnt = ncnt;
    } else {
      int lo = w[i], hi = w[i+1];
      while(hi - lo > 1) {
        int md = (lo + hi) / 2;
        dif = md - w[i];
        ncnt = cnt + dif * imos[i];
        if(ncnt < K) {
          lo = md;
        } else {
          hi = md;
        }
      }
      printf("%d\n", lo);
      return 0;
    }
  }
  puts("-1");
  return 0;
}