#include <bits/stdc++.h>

using namespace std;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int N, D;
  cin >> N >> D;
  vector<int> T(N), K(N);
  for (int i = 0; i < N; i++) {
    cin >> T[i] >> K[i];
  }
  vector<vector<int64_t>> dp(N + 1, vector<int64_t>(2, 0));
  dp[0][1] = -(1 << 28);
  for (int i = 0; i < N; i++) {
    dp[i + 1][0] = max(dp[i][0], dp[i][1] - D) + T[i];
    dp[i + 1][1] = max(dp[i][1], dp[i][0] - D) + K[i];
  }
  cout << max(dp[N][0], dp[N][1]) << '\n';
  return 0;
}