#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using T = tuple<int, int, int>;
#define al(a) a.begin(), a.end()
#define ral(a) a.rbegin(), a.rend()
#define sz(a) (int)a.size()
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = 1; i <= (n); ++i)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
#define db(a, b) cout << #a << ": " << a << " " << #b << ": " << b << endl;

int main() {
  int n, d;
  cin >> n >> d;
  vector<vector<int>> p(n);
  vector a(n, vector<int>(2));
  rep(i, n) rep(j, 2) { cin >> a[i][j]; }

  auto chmax = [&](ll &a, ll b) { a = max(a, b); };
  const ll INF = 1e18;
  vector<ll> dp(2, -INF);
  dp[0] = 0;
  rep(i, n) {
    vector<ll> p(2, -INF);
    swap(dp, p);
    rep(j, 2) rep(k, 2) {
      if (j == k)
        chmax(dp[j], p[k] + a[i][j]);
      else
        chmax(dp[j], p[k] + a[i][j] - d);
    }
  }
  ll ans = max(dp[0], dp[1]);
  cout << ans << endl;
  return 0;
}