#include <bits/stdc++.h>
#define ft first
#define sc second
#define lb lower_bound
#define ub upper_bound
#define pb push_back
#define pt(sth) cout << sth << "\n"
#define chmax(a, b) {if(a<b) a=b;}
#define chmin(a, b) {if(a>b) a=b;}
#define moC(a, s, b) (a)=((a)s(b)+MOD)%MOD
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
static const ll INF=1e18;
static const ll MAX=1e5+7;
static const ll MOD=1e9+7;
ll max(ll a, ll b) {return a>b?a:b;}
ll min(ll a, ll b) {return a<b?a:b;}

ll N, K;
ll a[MAX], b[MAX];

ll C(ll x) {
  ll cnt=0;
  ll i=0;
  for(i=0; i<N; i++) {
    cnt+=min(b[i]-a[i]+1, max(0, x-a[i]+1));
  }
  return cnt>=K;
}

int main(void) {
  cin >> N >> K;
  ll i;
  
  for(i=0; i<N; i++) {
    cin >> a[i] >> b[i];
  }
  
  ll l=0, r=INF;
  while(l+1<r) {
    ll mid=(l+r)/2;
    if(C(mid)) r=mid;
    else l=mid;
  }
  
  pt(l+1);
  
}