"""

同じ2種類のみを繰り返すのが最適？
最後の方は、そうとは限らない

二分探索か？
最低を下回らない範囲内で、一番跳ね返りが大きいのを選ぶのが最適

"""

import sys
from sys import stdin
import math

N,D = map(int,stdin.readline().split())

PQ = []
PG = []

for i in range(N):
    P,Q = map(int,stdin.readline().split())
    PQ.append((P,Q))
    PG.append( (P,Q-P) )

PG.sort()

r = 0
l = -10**9

while r-l != 1:

    m = (l+r)//2

    now = 0
    last = None

    able = True

    for day in range(D):

        nind = None
    
        for i in range(N):
            p,g = PG[i]
            if now - p >= m and i != last and (nind == None or PG[nind][1] < g):
                nind = i
            if now - p < m:
                break

        if nind == None:
            able = False
            break

        last = nind
        now += PG[nind][1]

    if able:
        l = m
    else:
        r = m

print (l)