#include <bits/stdc++.h>
#define rep(i, l, r) for (int i = (l); i < (r); i++)
using namespace std;

typedef long long ll;

int main() {
    int N, D;
    cin >> N >> D;
    vector<ll> P(N), Q(N);
    rep(i, 0, N) cin >> P[i] >> Q[i];
    ll ans = -1e18;
    rep(i, 0, N) {
        rep(j, 0, i) {
            if (Q[i] - P[i] + Q[j] - P[j] >= 0) {
                ans = max({ans, -Q[i], -Q[i] + P[i] - Q[j]});
            } else {
                ll x = (- Q[i] + P[i] - Q[j] + P[j]) * (D - 1);
                ans = max({ans, x - Q[i], x - Q[i] + P[i] - Q[j]});
            }
        }
    }
    cout << ans << endl;
}