#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream si("carnati.in");
ofstream so("carnati.out");

int d[2005];
pair<int,int> v[2005];

int main()
{
    int sol=0;
    int n,c;
    si>>n>>c;
    for(int i=1;i<=n;i++)
        si>>v[i].first>>v[i].second;
    sort(v+1,v+n+1);
    v[0].first=v[1].first-1;
    for(int j=1;j<=n;j++)
        for(int i=1;i<=n;i++)
        {
            d[i]=0;
            if(v[j].second<=v[i].second)
                d[i]=v[j].second;
            d[i]=max(d[i]+d[i-1]-(v[i].first-v[i-1].first)*c,d[i]-c);
            sol=max(sol,d[i]);
        }
    so<<sol;
    return 0;
}