#include<fstream>
#include<algorithm>
using namespace std;
ifstream in("carnati.in");
ofstream out("carnati.out");
struct client
{
    int p,t;
};
client v[2001];
int n , m , i , j ,s , maxi,local , old,timp;
bool comp(client a , client b)
{
    if(a.t<b.t)
        return 1;
    return 0;
}
int rezultat(int pretClient,int pretCarnat)
{
    if(pretCarnat<=pretClient)
        return pretCarnat;
    return 0;
}
int calcTimp( int timpAnterior , int timpCurent)
{
    return -(timpCurent-timpAnterior-1)*m;
}
int main()
{
    in>>n>>m;
    for(i=1;i<=n;i++)
        in>>v[i].t>>v[i].p;

    sort(v+1,v+1+n,comp);
    v[0].t=v[1].t-1;
    for(i=1;i<=n;i++)
    {
        s=0;
        old=v[1].t;
        for(j=1;j<=n;j++)
        {
            //calculare rezultat
            timp  = calcTimp( v[j-1].t , v[j]. t )  ;
            local = rezultat( v[j].p  , v[i].p )  ;
            //verificare
            if(s + timp > 0 )
            {
                //adunare
                s=s + local + timp -m;
                if(s>maxi)
                    maxi=s;
            }
            else
            {
                s=local-m;
            }
        }
    }
    out<<maxi;
}