#include <fstream>
#include <algorithm>

using namespace std;
ifstream fi("carnati.in");
ofstream fo("carnati.out");
pair <int,int> V[2001];
int n,i,j,pret,profit,s,c,maxim;
int main()
{
    fi>>n>>c;
    for(i=1;i<=n;i++)
        fi>>V[i].first>>V[i].second;
    sort(V+1,V+n+1);
    for(i=1;i<=n;i++)
    {
        pret=V[i].second;
        s=0;
        for(j=1;j<=n;j++)
        {
            s=max(0,s-c*(V[j].first-V[j-1].first));
            if(pret<=V[j].second)
                s+=pret;
            maxim=max(s-c,maxim);
        }
        profit=max(maxim,profit);
    }
    fo<<profit;
    fi.close();
    fo.close();
    return 0;
}