#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  int n, d;
  cin >> n >> d;
  constexpr ll INF = 1e18;
  vector<pair<ll, ll>> dp(n + 1, { -INF, -INF });
  dp[0].first = 0;
  rep(i, n) {
    int t, k;
    cin >> t >> k;
    dp[i + 1].first = max(dp[i].first + t, dp[i].second - d + t);
    dp[i + 1].second = max(dp[i].first - d + k, dp[i].second + k);
    // cerr << format("{}: {}\n", i, dp[i + 1]);
  }
  cout << max(dp[n].first, dp[n].second) << '\n';
  return 0;
}