#include <fstream>
#include <utility>
#include <algorithm>

using namespace std;

ifstream f("carnati.in");
ofstream g("carnati.out");

int n, c,sol,D[1<<11];
pair<int,int>v[1<<11];

int main()
{ int i, j, sol=0;

    f>>n>>c;
    for(i=1;i<=n;i++)
        f>>v[i].first>>v[i].second;
    sort(v+1,v+n+1);
    v[0].first=v[1].first-1;
    for(j=1;j<=n;j++)
        for(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]);
    }
  g<<sol;
}