def shunyu(N, D, TK):
    '''shunyu()はリスト。
    idx=0はN日目に東京で働いた場合の最大収入
    idx=1はN日目に京都で働いた場合の最大収入'''
    if N == 1:
        result = [TK[0][0], TK[0][1] - D]
        return result
    bef_shunyu = shunyu(N - 1, D, TK)
    t_li = [0, 0]
    k_li = [0, 0]
    t_li[0] = bef_shunyu[0] + TK[N - 1][0]
    t_li[1] = bef_shunyu[1] + TK[N - 1][0] - D
    k_li[0] = bef_shunyu[0] + TK[N - 1][1] - D
    k_li[1] = bef_shunyu[1] + TK[N - 1][1]
    result = [max(t_li), max(k_li)]
    return result

def hantei(N, D, TK):
    li = shunyu(N, D, TK)
    return max(li)

def main():
    N, D = map(int, input().split())
    TK = [[] for x in range(N)]
    for n in range(N):
        TK[n] = list(map(int, input().split()))
    print(hantei(N, D, TK))

if __name__ == '__main__':
    main()