import sys

sys.setrecursionlimit(10**7)
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int,sys.stdin.readline().rstrip().split())
def LI(): return list(map(int,sys.stdin.readline().rstrip().split()))
def LI2(): return list(map(int,sys.stdin.readline().rstrip()))
def S(): return sys.stdin.readline().rstrip()
def LS(): return list(sys.stdin.readline().rstrip().split())
def LS2(): return list(sys.stdin.readline().rstrip())


N,K = MI()
AB = [tuple(MI()) for _ in range(N)]


def f(x):  # x円以下の硬貨の枚数
    res = 0
    for i in range(N):
        A,B = AB[i]
        res += max(0,min(x-A+1,B-A+1))
    return res


ng = 0
ok = 10**9
while ng+1 < ok:
    mid = (ok+ng)//2
    if f(mid) >= K:
        ok = mid
    else:
        ng = mid

print(ok)