#include <iostream>
#include <stdio.h>
#include <cstring>
#include <algorithm>
#define NMax 2003
#define INFINIT 9999999999
using namespace std;
struct Client
{
    int timp;
    int bani;
    int chelt;
}v[NMax];
int D[NMax],sol=-INFINIT,C,N;

bool comp(Client x,Client y)
{
    return x.timp<y.timp;
}

void Dinamic()
{
    sort(v+1,v+N+1,comp);
    v[0].timp=v[1].timp-1;
    for(int i=1;i<=N;i++)
        v[i].chelt=(v[i].timp-v[i-1].timp)*C;

    for(int i=1;i<=N;i++)
    {
        memset(D,0,sizeof(D));
        int pret=v[i].bani;
        int Max=-INFINIT;

        for(int j=1;j<=N;j++)
        {
            if(v[j].bani>=pret)
                D[j]=max(pret-C,D[j-1]+pret-v[j].chelt);
            else
                D[j]=max(-C,D[j-1]-v[j].chelt);
            Max=max(Max,D[j]);
        }
        sol=max(sol,Max);
    }
}

int main()
{
    freopen("carnati.in","r",stdin);
    freopen("carnati.out","w",stdout);
    scanf("%d%d",&N,&C);
    for(int i=1;i<=N;i++)
        scanf("%d%d",&v[i].timp,&v[i].bani);
    Dinamic();
    printf("%d",sol);
}