n,d=map(int,input().split())
tko=[0]*n
kyo=[0]*n
tk=[list(map(int,input().split())) for _ in range(n)]
tko[0]=tk[0][0]
kyo[0]=tk[0][1]-d
for i in range(1,n):
    t,k=tk[i]
    tko[i]=max(tko[i-1],kyo[i-1]-d)+t
    kyo[i]=max(kyo[i-1],tko[i-1]-d)+k
ans=max(tko[-1],kyo[-1])
print(ans)