#include <bits/stdc++.h>
#define Dim 2004
using namespace std;
ifstream f("carnati.in");
ofstream g("carnati.out");
int N,C,Ans[Dim],ans,prec;


struct point { int time,pay; } V[Dim];

bool cmp(point X,point Y)
{
    if(X.time<Y.time) return 1;
    else return 0;
}

int main()
{
    f>>N>>C;
    for(int i=1;i<=N;i++) f>>V[i].time>>V[i].pay;
    sort(V+1,V+N+1,cmp);
    for(int i=1;i<=N;i++)
    {
        int pret=V[i].pay;
        for(int j=1;j<=N;j++) Ans[j]=0;
        prec=0;
        for(int j=1;j<=N;j++)
        {
            if(V[j].pay>=pret)
            {
                Ans[j]=pret-C;
                if(Ans[j]<=0) Ans[j]=0;
                Ans[j]=max(Ans[j],Ans[prec]-C*(V[j].time-V[prec].time)+pret);
                ans=max(ans,Ans[j]);
                prec=j;
                //cout<<i<<" "<<j<<" "<<Ans[j]<<'\n';
            }

        }

    }
    g<<ans;
    return 0;
}