import sys

sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
dij = [(0, 1), (1, 0), (0, -1), (-1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = 1 << 63
# md = 998244353
md = 10**9+7

n, d = LI()
pq = LLI(n)

def ok(m):
    mx1, mx2 = 0, -inf
    i1 = -1
    for _ in range(d):
        nmx1 = nmx2 = -inf
        ni1 = -1
        for i, (p, q) in enumerate(pq):
            pre = mx1
            if i == i1: pre = mx2
            if pre-p < m: continue
            cur = pre-p+q
            if cur > nmx1:
                nmx2 = nmx1
                nmx1 = cur
                ni1 = i
            elif cur > nmx2:
                nmx2 = cur
        if nmx1 == -inf: return False
        mx1, mx2, i1 = nmx1, nmx2, ni1
    return True

l, r = -10**9, 0
while l+1 < r:
    m = (l+r)//2
    if ok(m): l = m
    else: r = m
print(l)