import sys
stdin=sys.stdin

ip=lambda: int(sp())
fp=lambda: float(sp())
lp=lambda:list(map(int,stdin.readline().split()))
tp=lambda:tuple(map(int,stdin.readline().split()))
sp=lambda:stdin.readline().rstrip()
yp=lambda:print('Yes')
np=lambda:print('No')

n,d=lp()
inf=float('inf')
dp=[[-inf for _ in range(2)]for _ in range(n)]
for i in range(n):
    now=lp()
    if i==0:
        dp[0][0]=now[0]
        dp[0][1]=now[1]-d
    else:
        dp[i][0]=max(dp[i-1][0]+now[0],dp[i-1][1]+now[0]-d)
        dp[i][1]=max(dp[i-1][1]+now[1],dp[i-1][0]+now[1]-d)

print(max(dp[-1][0],dp[-1][1]))