#include <bits/stdc++.h>

using namespace std;
ifstream f("carnati.in");
ofstream g("carnati.out");
int n,c;
struct wow
{
    int t,y;
}v[2005];
vector <int> vec[2005];
int i,sol,pret,sum,maxim,j,k;
int main()
{
    f>>n>>c;
    for (i=1;i<=n;i++)
    {
        f>>v[i].t>>v[i].y;
        vec[v[i].t].push_back(v[i].y);
        maxim=max(maxim,v[i].t);
    }
    sol=0;
    for (i=1;i<=n;i++)
    {
        pret=v[i].y;
        sum=0;
        for (j=0;j<=maxim;j++)
        {
            for (k=0;k<vec[j].size();k++)
            {
                if (pret<=vec[j][k])
                {
                    sum=sum+pret;
                }
            }
            sum=sum-c;
            if (sum>sol)
            {
                sol=sum;
            }
            if (sum<0)
            {
                sum=0;
            }
        }
    }
    g<<sol;
    return 0;
}