#include<cstdio>
#include<algorithm>
using namespace std;
struct eu{int t,val;};
eu v[2001];
int i,j,sum,n,m,k,cate,p1;
bool sorting(eu a,eu b)
    {
    return a.t<b.t;
    }
int valmax,c;
int main ()
{
freopen("carnati.in","r",stdin);
freopen("carnati.out","w",stdout);
scanf("%d%d",&n,&c);
for(i=1;i<=n;i++)
    scanf("%d%d",&v[i].t,&v[i].val);
sort(v+1,v+n+1,sorting);
for(int qq=1;qq<=n;qq++)
    {
    int x=v[qq].val;
    int sum=0,p1=1,h=0;
    for(i=1;i<=n;i++)
        {
        if(v[i].val>=x)
            sum=max(sum+x-c*(v[i].t-v[i-1].t),x-c);
        else
            sum=max(h,sum-c*(v[i].t-v[i-1].t));
        if(sum>valmax)
            valmax=sum;
        }
    }
printf("%d",valmax);
return 0;
}