#include <fstream>
#include <algorithm>

using namespace std;

ifstream fin("carnati.in");
ofstream fout("carnati.out");
int N,C,sol;
pair <int,int> v[2005];
int D[2005];
int main(){
    fin>>N>>C;
    for(int i=1;i<=N;i++)
        fin>>v[i].first>>v[i].second;
    sort(v+1,v+N+1);
    v[0].second=v[1].second-1;
    for(int j=1;j<=N;j++)
        for(int 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]);
        }
    fout<<sol;
    fin.close();fout.close();
    return 0;
}