#include <cstring>
#include <fstream>
#include <algorithm>
#include <vector>

using namespace std;

int N, C;
int money[2002], maxtime;
vector<int> V[1502];
int maxS[1502];
int result;

int main()
{
    ifstream fin("carnati.in");
    ofstream fout("carnati.out");

    fin >> N >> C;
    for (int i = 1, time; i <= N; ++i)
    {
        fin >> time >> money[i];

        maxtime = max(maxtime, time);
        V[time].push_back(money[i]);
    }

    for (int i = 1; i <= N; ++i) // daca pretul este i
    {
        memset(maxS, 0, sizeof(maxS));
        for (int j = 0; j <= maxtime; ++j)
        {
            int add = 0;
            for (vector<int>::iterator it = V[j].begin(); it != V[j].end(); ++it)
                if (*it >= money[i])
                    add += money[i];

            if (j == 0) maxS[j] = add - C;
            else        maxS[j] = max(add - C, maxS[j - 1] + add - C);

            result = max(result, maxS[j]);
        }
    }

    fout << result;

    fin.close();
    fout.close();
}