N,D = list(map(int,input().split()))
food = []
for _ in range(N):
    p,q = list(map(int,input().split()))
    food.append((p,q))
food.sort(key=lambda x:x[1] - x[0])

def check(n):
    pre = -1
    now = 0
    for _ in range(D):
        flag = False
        for i in range(N):
            if(pre == i):continue
            p,q = food[i]
            if(now - p < n):continue
            now += -p + q
            pre = i
            flag = True
            break
        if(not flag):return False
    return True            

ok,ng = -10**9,10**9
while(ng - ok > 1):
    mid = (ok+ng)//2
    if(check(mid)):
        ok = mid
    else:
        ng = mid
print(ok)