#include <bits/stdc++.h>
#define N 2001
struct carnat
{
    int timp, pret;
} v[N];

bool cmp(carnat a, carnat b)
{
    if(a.timp < b.timp)
        return true;
    else
        return false;
}

int main()
{
    freopen("carnati.in", "r", stdin);
    freopen("carnati.out", "w", stdout);
    int n, c, i, smax, s, x, j, max;

    scanf("%d%d", &n, &c);
    for (i = 1; i <= n; i++)
        scanf("%d%d", &v[i].timp, &v[i].pret);

    std::sort(v + 1, v + n + 1, cmp);

    smax = -INFINITY;
    for (i = 1; i <= n; i++)
    {
        s = -c;
        for (j = 1; j <= n; j++)
        {
            if (v[i].pret <= v[j].pret)
            {
                s = s + v[i].pret - (v[j].timp - v[j - 1].timp) * c;
                if (s < 0)
                    s = -c;
                if (s > smax)
                    smax = s;
            }
        }
    }
    printf("%d", smax);
    return 0;
}