N,D = map(int,input().split())
T,K = [0 for i in range(N)],[0 for i in range(N)]
for i in range(N):
	T[i],K[i] = map(int,input().split())

dp = [[0,0] for i in range(N + 1)]

for i in range(N):
	for j in range(2):
		if j == 0:
			dp[i+1][j] = max(dp[i+1][j],dp[i][j] + T[i])
			dp[i+1][1] = max(dp[i+1][1],dp[i][j] + K[i] - D)
		elif i != 0:
			dp[i+1][j] = max(dp[i+1][j],dp[i][j] + K[i])
			dp[i+1][0] = max(dp[i+1][0],dp[i][j] + T[i] - D)
print(max(dp[N][0],dp[N][1]))