mod = 1000000007
eps = 10**-9


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

    N, D = map(int, input().split())
    T = [0] * N
    K = [0] * N
    for i in range(N):
        t, k = map(int, input().split())
        T[i] = t
        K[i] = k

    dp = [[0] * 2 for _ in range(N+1)]
    dp[0][1] = -(10**10)
    for i in range(N):
        dp[i+1][0] = max(dp[i][0] + T[i], dp[i][1] + T[i] - D)
        dp[i+1][1] = max(dp[i][1] + K[i], dp[i][0] + K[i] - D)
    print(max(dp[-1]))


if __name__ == '__main__':
    main()