from heapq import nlargest

N, D = map(int, input().split())
PQs = [tuple(map(int, input().split())) for _ in range(N)]

# dp[最後の料理] = 元気度
# 元気度の下界を二分探索

# new_dp[料理] = max dp[他の料理] + 差分 (下界に違反しない場合)

INF = 10 ** 9

def isok(lb):
    dp = [0] * N
    for _ in range(D):
        new_dp = [-INF] * N
        M1, M2 = nlargest(2, dp)
        if M1 == -INF:
            return False
        for i, (P, Q) in enumerate(PQs):
            M = M1 if (M1 != dp[i]) else M2
            if M - P >= lb:
                new_dp[i] = M - P + Q
        dp = new_dp
    return max(dp) > -INF


ok = -INF
ng = 1
while ng - ok > 1:
    mid = (ng + ok) // 2
    if isok(mid):
        ok  = mid
    else:
        ng = mid

print(ok)