#include<fstream>
#include<algorithm>
using namespace std;
pair<int,int> v[2005];
int main()
{
    ifstream cin("carnati.in");
    ofstream cout("carnati.out");
    int n,k,nr,val,s,ans=0,t;
    cin>>n>>k;
    for(int i=1;i<=n;++i)
        cin>>v[i].first>>v[i].second;
    sort(v+1,v+n+1);
    for(int i=1;i<=n;++i)
    {
         val=v[i].second;
          s=0;
        for(int j=1;j<=n;++j)
        {
            if(v[j].second>=val)
                nr=val;
            else
                nr=0;
                t=v[j].first-v[j-1].first;
                s=max(s+nr-k*t,nr-k);
                ans=max(ans,s);
        }
    }
    cout<<ans;
}