mod = 1000000007
eps = 10**-9


def main():
    import sys
    input = sys.stdin.readline

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

    ok = -10 ** 9
    ng = 0
    mid = (ok + ng) // 2
    while ng - ok > 1:
        prev = -1
        cur = 0
        flg = 1
        for d in range(D):
            i_best = -1
            cur_best = -10 ** 9
            for i in range(N):
                if cur - P[i] < mid:
                    continue
                if prev == i:
                    continue
                cur_new = cur - P[i] + Q[i]
                if cur_new > cur_best:
                    cur_best = cur_new
                    i_best = i
            if i_best == -1:
                flg = 0
                break
            prev = i_best
            cur = cur_best
        if flg == 0:
            ng = mid
        else:
            ok = mid
        mid = (ok + ng) // 2
    print(ok)


if __name__ == '__main__':
    main()