#include <iostream>
#include <fstream>
#include <vector>
using namespace std;
int p[2001],n,c,profit;
vector<int> t[1501];
void rez(int cost)
{
    int i,acumulat=0,j;
    for (i=0;i<=1500;i++)
    {
        if (acumulat<0)
        {
            acumulat=0;
        }
        for (j=0;j<t[i].size();j++)
        {
            if (cost<=t[i][j])
            acumulat+=cost;
        }
        acumulat-=c;
        if (profit<acumulat)
        {
            profit=acumulat;
        }
    }
}
int main()
{
    ifstream in("carnati.in");
    ofstream out("carnati.out");
    int i,x;
    in>>n;
    in>>c;
    for (i=1;i<=n;i++)
    {
        in>>x;
        in>>p[i];
        t[x].push_back(p[i]);
    }
    for (i=1;i<=n;i++)
    {
        rez(p[i]);
    }
    out<<profit;
}