n, d = map(int, input().split())
dp = [[0, 0] for _ in range(n + 1)]
dp[0][1] = -1 << 30

for i in range(1, n + 1):
    t, k = map(int, input().split())
    dp[i][0] = max(dp[i - 1][0] + t, dp[i - 1][1] - d + t)
    dp[i][1] = max(dp[i - 1][1] + k, dp[i - 1][0] - d + k)

print(max(dp[-1]))