N, D = map(int, input().split())

dp = [[0, 0] for _ in range(N+1)]
dp[0] = [0, -D]

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] + T - D)
    dp[i][1] = max(dp[i-1][0] + K - D, dp[i-1][1] + K)

print(max(dp[N]))