#include <fstream>
#include <algorithm>

using namespace std;
ifstream cin("carnati.in");
ofstream cout("carnati.out");
int n,c,i,j,mx,vl;
pair <int,int> v[2005];
int p[2005];
int main()
{
    cin>>n>>c;
    for(i=1;i<=n;i++)
        cin>>v[i].first>>v[i].second;
    sort(v+1,v+n+1);
    vl=0;
    v[0].first=v[i].second=p[0]=-1;
    for(i=1;i<=n;i++)
    {
        vl=v[i].second;
        for(j=1;j<=n;j++)
        {
            if(v[j].second>=vl)
                p[j]=max(p[j-1]-(v[j].first-v[j-1].first)*c+vl,vl-c);
            else
                p[j]=max(p[j-1]-(v[j].first-v[j-1].first)*c,-c);
            mx=max(mx,p[j]);
        }
    }
    cout<<mx;
    return 0;
}