#include<cstdio>
#include<algorithm>
using namespace std;
struct eu{int t,val;};
eu v[2001];
int i,j,sum,n,m,k,cate,p1;
int valmax,c;
bool sorting(eu a,eu b)
    {
    if(a.t<b.t||a.t==b.t&&a.val>b.val)
        return 1;
    return 0;
    }
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;
    if(x-c>valmax)
        valmax=x-c;
    for(i=1;i<=n;i++)
        {
        if(v[i].val>=x)
            sum+=x;
        while(v[i].t==v[i+1].t)
            {
            i++;
            if(v[i].val>=x)
                sum+=x;
            }
        if(sum-(v[i].t-v[p1].t+1)*c>valmax)
            valmax=sum-(v[i].t-v[p1].t+1)*c;
        if(sum-(v[i].t-v[p1].t+1)*c<0)
            {
            if(v[i].val<x)
                {
                sum=0;
                p1=i+1;
                }
            else
                {
                sum=x;
                p1=i;
                }
            }
        }
    }
printf("%d",valmax);
return 0;
}