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

#define reps(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n)   reps(i,0,n)
#define all(x) (x).begin(),(x).end()
#define INF (1000000000)
#define MOD (1000000007)
#define PI (acos(-1))

ll N,K;
ll A[100010],B[100010];
bool isOK(ll x){//x円以下のコインがk枚以上存在するか
  ll cnt=0;
  rep(i,N){
    cnt += max((ll)0,(min(B[i],x)-A[i])+1);
    if(cnt >= K)return true;
  }
  return false;
}

int main(){
  cin >> N >> K;
  rep(i,N){
    cin >> A[i] >> B[i];
  }

  ll ok = 1e18, ng = 0;
  while(abs(ok-ng)>1){
    ll mid = (ok+ng)/2;
    if(isOK(mid)){
      ok = mid;
    }else{
      ng = mid;
    }
  }
  cout << ok << endl;
}