N, D = map(int, input().split())
dp = [[0] * 2 for _ in range(N + 1)]
dp[0][1] = -D

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

# print(dp)
ans = max(dp[-1])
print(ans)