n, d = map(int,input().split())
job = []
for _ in range(n):
    t, k = map(int,input().split())
    job.append((t,k))
    
#0:東京，1:京都
dp = [[0]*2 for _ in range(n)]
dp[0][0] = job[0][0]
dp[0][1] = job[0][1]-d
for i in range(1,n):
    dp[i][0] = max(dp[i-1][0], dp[i-1][1]-d)+job[i][0]
    dp[i][1] = max(dp[i-1][0]-d, dp[i-1][1])+job[i][1]
    
print(max(dp[n-1][0],dp[n-1][1]))

#print(dp)