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

using namespace std;

ifstream fin("carnati.in");
ofstream fout("carnati.out");

int n,c,a[2001],x;
pair <int,int > v[2001];

int main()
{
    fin >> n >> c;
    int i,j;
    int rasp=-1;
    for(i=1;i<=n;i++)
    {
        fin >> v[i].first >> v[i].second; /// first=timp(Ti) , second=pret(Pi)
    }
    sort(v+1,v+n+1);
    v[0].first=v[1].first-1;
    for(i=1;i<=n;i++)
    {
        for(j=1;j<=n;j++)
        {
            a[j]=0;
            if(v[i].second<=v[j].second) a[j]=v[i].second;
            x=(v[j].first-v[j-1].first)*c;
            a[j]=max(a[j]+a[j-1]-x,a[j]-c);
            if(a[j]>rasp)
                rasp=a[j];
        }
    }
    fout << rasp;
    return 0;
}