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

using ll = long long;
using P = pair<int, int>;

const int INF = 1e9;
const int MOD = 1e9 + 7;

int dp[110][2]; // N日目に東京 or 京都でもらえる最大の収入を格納

int main() {
  int N, D;
  cin >> N >> D;
  int t[N], k[N];
  for (int i = 0; i < N; i++) cin >> t[i] >> k[i];

  dp[0][0] = t[0];
  dp[0][1] = k[0] - D;
  for (int i = 1; i < N; i++) {
    dp[i][0] = t[i] + max(dp[i-1][0], dp[i-1][1] - D);
    dp[i][1] = k[i] + max(dp[i-1][0] - D, dp[i-1][1]);
  }

  cout << max(dp[N-1][0], dp[N-1][1]) << endl;
  return 0;
}