#include <fstream>
#include <algorithm>
#define NMAX 2000
#define TMAX 1500

using namespace std;

typedef long long ll;
int timp[TMAX];

struct carnati
{
    int t, p;
};

carnati v[NMAX];

bool cmp(carnati a, carnati b)
{
    return (a.p > b.p);
}

int main()
{
    ifstream fin("carnati.in");
    ofstream fout("carnati.out");
    int n, C;
    fin >> n >> C;
    for (int i = 0; i < n; i++)
    {
        fin >> v[i].t >> v[i].p;
    }
    sort(v, v + n, cmp);
    int nr = 0, maxx = -1;
    for (int i = 0; i < n; i++)
    {
        timp[v[i].t]++;
        nr = 0;
        for (int j = 1; j <= TMAX; j++)
        {
            nr = timp[j] * v[i].p - C + max(nr, 0);
            if (nr > maxx)
            {
                maxx = nr;
            }
        }
    }
    fout << maxx;

    return 0;
}