INF = 10 ** 18
n, d = map(int, input().split())
dp = [[0] * (n + 1) for _ in range(2)]
dp[0][0] = 0
dp[1][0] = -INF
for i in range(n):
    t, k = map(int, input().split())
    dp[0][i + 1] = max(dp[0][i] + t, dp[1][i] + t - d)
    dp[1][i + 1] = max(dp[0][i] + k - d, dp[1][i] + k)
print(max(dp[0][n], dp[1][n]))