#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("carnati.in");
ofstream fout("carnati.out");
int n,i,j,c,f[2005],s,sol;
pair <int,int> v[2005];
int main(){
    fin>>n>>c;
    for(i=1;i<=n;i++){
        fin>>v[i].first>>v[i].second;
    }
    sort(v+1,v+n+1);
    for(i=1;i<=n;i++){
        for(j=1;j<=n;j++){
            if(v[i].second<=v[j].second){
                f[j]=v[i].second;
            }
            else{
                f[j]=0;
            }
            f[j]=max(f[j]+f[j-1]-(v[j].first-v[j-1].first)*c,f[j]-c);
            sol=max(sol,f[j]);
        }
    }
    fout<<sol<<"\n";

    return 0;
}