n,d = map(int,input().split())
z = [list(map(int,input().split())) for i in range(n)]

dp = [[-float("Inf"), -float("Inf")] for i in range(n+1)]
# 東京, 京都
dp[0][0] = 0

for i,[t,k] in enumerate(z):
    #東京から
    dp[i+1][0] = max(dp[i+1][0], dp[i][0] + t)
    dp[i+1][1] = max(dp[i+1][1], dp[i][0] + k - d)

    #京都から
    dp[i+1][0] = max(dp[i+1][0], dp[i][1] + t -d)
    dp[i+1][1] = max(dp[i+1][1], dp[i][1] + k)

print(max(dp[-1]))