#include <fstream>
#include <limits.h>
#include <algorithm>
using namespace std;

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

const long long NMAX = 2001;

long long max(long long a, long long b)
{
    return (a > b) ? a : b;
}

long long min(long long a, long long b)
{
    return (a < b) ? a : b;
}

struct carnati
{
    long long t, p;
};

inline bool cmp(carnati a, carnati b)
{
    return a.t < b.t;
}

carnati a[NMAX];

int n, c;
long long maxi, nr;

int main()
{
    fin >> n >> c;

    for (int i = 1; i <= n; ++i)
        fin >> a[i].t >> a[i].p;

    sort(a + 1, a + n + 1, cmp);

    for (int i = 1; i <= n; ++i)
    {
        long long x = a[i].p;
        long long t = 0, p = -2000000001;

        for (int j = 1; j <= n; ++j)
        {
            if (a[j].p >= x)
            {
                if (p + x - (a[j].t - t) * c < x - c) p = x - c;
                else
                    p += x - (a[j].t - t) * c;
                t = a[j].t;
                maxi = max(maxi, p);
            }
        }
    }

    fout << maxi << "\n";
    return 0;
}