n,x=map(int,input().split())
a,b=map(int,input().split())

b-=x
for i in range(n-1):
    lt,lk=a,b
    nt,nk=map(int,input().split())
    a=max(lt+nt,lk-x+nt)
    b=max(lk+nk,lt-x+nk)
print(max(a,b))