#include <iostream>
#include <algorithm>

using namespace std;

struct client {
    long long t, p;
};

bool cmp(client &a, client &b) {
    return a.t < b.t;
}

client v[2001];
long long pret[2001];

int main() {
    cin.tie(nullptr)->sync_with_stdio(false);
    freopen("carnati.in", "r", stdin);
    freopen("carnati.out", "w", stdout);
    int n;
    long long c;
    cin >> n >> c;
    for (int i = 0; i < n; i++) {
        cin >> v[i].t >> v[i].p;
        pret[i] = v[i].p;
    }
    sort(v, v + n, cmp);
    sort(pret, pret + n);
    long long mx_p = 0;
    for (int i = 0; i < n; i++) {
        long long p = pret[i];
        if (i > 0 && pret[i] == pret[i - 1]) {
            continue;
        }
        for (int j = 0; j < n; j++) {
            long long cnt_clienti = 0;
            for (int k = j; k < n; k++) {
                if (v[k].p >= p) {
                    cnt_clienti++;
                }
                long long timp_total = (v[k].t - v[j].t) + 1;
                long long profit = cnt_clienti * p - timp_total * c;
                mx_p = max(mx_p, profit);
            }
        }
    }
    cout << mx_p;
    return 0;
}