#include<bits/stdc++.h>
using namespace std;

using ll = long long;
const int INF = 1e9;
const ll inf = 1LL<<60;

int main() {
  int n, d; cin >> n >> d;
  vector<ll> p(n), q(n);
  for (int i=0; i<n; i++) cin >> p[i] >> q[i];
  ll ans = -INF;
  for (int i=0; i<n; i++) for (int j=0; j<n; j++) if (i != j) {
    ll fd = q[i] - p[i], sd = q[j] - p[j];
    if (fd + sd >= 0) ans = max(ans, min(-p[i], fd-p[j]));
    else {
      ll last = (fd + sd)*(d/2);
      if (d & 1) last += fd;
      if (d & 1) {
        ans = max(ans, min(last - q[i], last - fd - q[j]));
      } else {
        ans = max(ans, min(last - q[j], last - sd - q[i]));
      }
    }
  }
  cout << ans << endl;
}