import sys; input = sys.stdin.buffer.readline
sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

#処理内容
def main():
	N, D = getlist()
	DP = [[0, -INF] for i in range(N + 1)]
	for i in range(N):
		T, K = getlist()
		DP[i + 1][0] = max(DP[i][0], DP[i][1] - D) + T
		DP[i + 1][1] = max(DP[i][1], DP[i][0] - D) + K

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

if __name__ == '__main__':
	main()