#include <iostream>
#include <fstream>
#include <algorithm>

using namespace std;

ifstream x("carnati.in");
ofstream y("carnati.out");

long long n,p,i,prof[2002],profit=-1,j;

struct elem
{
    int h,c;
}v[2002];

bool cmp(elem a, elem b)
{
    return a.h<b.h;
}

int main()
{
    x>>n>>p;
    for(i=1;i<=n;i++)
        x>>v[i].h>>v[i].c;
    sort(v+1,v+n+1,cmp);
    v[0].h=v[1].h-1;
    for(i=1;i<=n;i++)
    {
        for(j=1;j<=n;j++)
        {
            prof[j]=0;
            if(v[i].c<=v[j].c)
                prof[j]=v[i].c;
            prof[j]=max(prof[j]+prof[j-1]-p*(v[j].h-v[j-1].h),prof[j]-p);
            if(prof[j]>profit)
                profit=prof[j];
        }
    }
    y<<profit;
    x.close();
    y.close();
    return 0;
}