N, D = map(int, input().split())
INF = 1 << 60

p = []
q = []
for _ in range(N):
    s, t = map(int, input().split())
    p.append(s)
    q.append(t)


def solve(x):
    dp = [[-INF] * N for _ in range(D)]
    for i in range(N):
        if -p[i] >= x:
            dp[0][i] = q[i] - p[i]
    for i in range(1, D):
        la = [-INF for _ in range(N + 1)]
        ra = [-INF for _ in range(N + 1)]
        for j in range(N):
            la[j + 1] = max(la[j], dp[i - 1][j])
            ra[j + 1] = max(ra[j], dp[i - 1][N - j - 1])
        for j in range(N):
            tmp = max(la[j], ra[N - j - 1])
            if tmp - p[j] >= x:
                dp[i][j] = tmp - p[j] + q[j]

    for i in range(N):
        if dp[D - 1][i] != -INF:
            return True
    return False


left = -(10 ** 8)
right = 0

while right - left > 1:
    mid = (right + left) // 2
    if solve(mid):
        left = mid
    else:
        right = mid

print(left)