#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) {
            ll x = - P[i] + Q[i] - P[j] + Q[j];
            if (x >= 0) {
                ans = max(ans, min(- P[i], - P[i] + Q[i] - P[j]));
            } else {
                ll y = x * (D - 1);
                ans = max(ans, min(y - P[i], y - P[i] + Q[i] - P[j]));
            }
        }
    }
    cout << ans << endl;
}