#include <bits/stdc++.h>
#define nmx 2005
#define sqr 317
using namespace std;
int n,c;
pair <int,int> v[nmx];
int main()
{
    ifstream f ("carnati.in");
    ofstream g ("carnati.out");
    f>>n>>c;
    for (int i=1; i<=n; i++)
        f>>v[i].first>>v[i].second;
    sort (v+1,v+n+1);
    int rsp=0;
    for (int l=1; l<=n; l++)
    {
        int cost=v[l].second,mx=0;
        if (cost<=c) continue;
        int now=0,prv=v[1].first-1;
        //cout<<v[l].second<<'\n';
        for (int i=1; i<=n; i++)
        {
            //cout<<v[i].first<<' ';
            if (v[i].second>=cost)
            {
                now=now-(v[i].first-prv-1)*c;
                if (now<0) now=0;
                now+=cost-c;
                prv=v[i].first;
                mx=max(mx,now);
            }
            //cout<<now<<'\n';
        }
        //cout<<v[l].second<<' '<<mx<<'\n'<<'\n';
        rsp=max(rsp,mx);
    }
    g<<rsp;
}