#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin ("carnati.in");
ofstream fout("carnati.out");
int n,i,j,C,sol,D[2002];
pair <int,int>V[2002];
int main()
{
    fin>>n>>C;
    for(i=1;i<=n;i++)
        fin>>V[i].first>>V[i].second;
    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++)
        {
            D[j]=0;
            if(V[i].second<=V[j].second)
                D[j]=V[i].second;
            D[j]=max(D[j]+D[j-1]-(V[j].first-V[j-1].first)*C,D[j]-C);
            sol=max(sol,D[j]);
        }
    fout<<sol;
    return 0;
}