N, D = map(int, input().split())
TK = [list(map(int, input().split())) for _ in range(N)]
dp = [[0, -10 ** 15] for _ in range(N + 1)]
for i, (t, k) in enumerate(TK):
    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(max(dp[N]))