#include <fstream>
#include <algorithm>
#define nmax 2005
#define t first
#define p second
using namespace std;
ifstream cin("carnati.in");
ofstream cout("carnati.out");
int n,c,sol;
pair<int,int>v[nmax];
signed main()
{
    cin>>n>>c;
    for(int i=1;i<=n;i++)
        cin>>v[i].t>>v[i].p;
    sort(v+1,v+n+1);
    for(int i=1;i<=n;i++){
        int pret=v[i].p,s=0,last=v[1].t-1;
        for(int j=1;j<=n;j++){
            if(s-c*(v[j].t-last)+pret*(v[j].p>=pret)<=0){
                s=0;
                last=v[j+1].t-1;
            }else{
                s=s-c*(v[j].t-last)+pret*(v[j].p>=pret);
                last=v[j].t;
            }
            sol=max(sol,s);
        }
    }
    cout<<sol;
    return 0;
}