# -*- coding: utf-8 -*-

import sys
import copy

sys.setrecursionlimit(1000000)


# input = sys.stdin.readline

# ~~~~~~~~~~~~~~~~~~~~~_(＾～＾ ｣ ∠)_~~~~~~~~~~~~~~~~~~~~~

def isGood(N, K, mid, A, B):
    sum = 0
    for i in range(N):
        sum += max(0, min(mid, B[i]) - A[i] + 1)
    return sum >= K


N, K = map(int, input().split())

A, B = [], []
for _ in range(N):
    a, b = map(int, input().split())
    A.append(a)
    B.append(b)

ng = 0
ok = int(1e11)

while abs(ok - ng) > 1:
    mid = (ok + ng) // 2
    if isGood(N, K, mid, A, B) :
        ok = mid
    else:
        ng = mid

print(ok)