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):
            best = -10**9
            best_i = []
            for i in range(N):
                if cur - P[i] < mid:
                    continue
                if i == prev:
                    continue
                cur_new = cur - P[i] + Q[i]
                if cur_new > best:
                    best_i = [i]
                    best = cur_new
                elif cur_new == best:
                    best_i.append(i)
            if not best_i:
                flg = 0
                break
            if best < cur:
                if len(best_i) > 1:
                    PP = []
                    for i in best_i:
                        PP.append((P[i], i))
                    PP.sort(key=lambda x: x[0])
                    if (D - d) & 1:
                        prev = PP[0][1]
                    else:
                        prev = PP[1][1]
                else:
                    prev = best_i[0]
            else:
                prev = best_i[0]
            cur = best
        if flg == 0:
            ng = mid
        else:
            ok = mid
        mid = (ok + ng) // 2
    print(ok)


if __name__ == '__main__':
    main()