def parse_input():
    try:
        N, D = map(int, input().split())
        if N <= 0 or D < 0:
            raise ValueError("N must be greater than 0 and D must be non-negative.")
        T, K = [], []
        for _ in range(N):
            t, k = map(int, input().split())
            if t < 0 or k < 0:
                raise ValueError("Job incomes must be non-negative.")
            T.append(t)
            K.append(k)
    except ValueError as e:
        print(f"Invalid input: {e}")
        exit(1)
    return N, D, T, K

def calculate_max_income(N, D, T, K):
    dp_tokyo = [0] * (N + 1)  # Dynamic programming table for staying in Tokyo
    dp_kyoto = [0] * (N + 1)  # Dynamic programming table for staying in Kyoto

    # Initialization: A's home is in Tokyo
    dp_tokyo[0] = 0
    dp_kyoto[0] = -D  # Subtract the cost of moving to Kyoto on the first day

    # Populate the DP tables
    for i in range(1, N + 1):
        dp_tokyo[i] = max(dp_tokyo[i - 1] + T[i - 1], dp_kyoto[i - 1] + K[i - 1] - D)
        dp_kyoto[i] = max(dp_kyoto[i - 1] + K[i - 1], dp_tokyo[i - 1] + T[i - 1] - D)

    return max(dp_tokyo[N], dp_kyoto[N])

# Main
if __name__ == "__main__":
    N, D, T, K = parse_input()
    max_income = calculate_max_income(N, D, T, K)
    print(max_income)