def main():
    N, D = list(map(int, input().split(" ")))
    Tn, Kn = [], []
    for _ in range(N):
        t, k = list(map(int, input().split(" ")))
        Tn.append(t)
        Kn.append(k)
    ans_list = [[0] * N for _ in range(2)]
    ans_list[0][0] = Tn[0]
    ans_list[1][0] = Kn[0] - D
    for j in range(1, N):
        try:
            ans_list[0][j] = max(ans_list[0][j-1] + Tn[j], ans_list[1][j-1] + Tn[j] - D)
            ans_list[1][j] = max(ans_list[1][j-1] + Kn[j], ans_list[0][j-1] + Kn[j] - D)
        except:
            raise
    print(max(ans_list[0][-1], ans_list[1][-1]))

if __name__ == "__main__":
    main()