N, D = map(int, input().split())
T = [0] * N
K = [0] * N
for i in range(N):
    T[i], K[i] = map(int, input().split())
dp = [[0, 0] for _ in range(N)]
for i in range(N):
    if i == 0:
        dp[i][0] = T[i]
        dp[i][1] = K[i] - D
    else:
        dp[i][0] = max(dp[i - 1][0] + T[i], dp[i - 1][1] + T[i] - D)
        dp[i][1] = max(dp[i - 1][0] + K[i] - D, dp[i - 1][1] + K[i])
print(max(dp[N - 1][0], dp[N - 1][1]))