#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;

typedef long long ll;
typedef pair<ll, ll> P;

#define MOD 1000000007
#define REP(i, N) for (int i = 0; i < N; ++i)
#define REP1(i, N) for (int i = 1; i <= N; ++i)
#define RREP(i, N) for (int i = N - 1; i >= 0; --i)
#define ALL(a) a.begin(), a.end()

ll dp[101][2];

int main() {
  int n;
  ll d;
  cin >> n >> d;
  ll t[n], k[n];
  REP(i, n) cin >> t[i] >> k[i];
  dp[1][0] = t[0];
  dp[1][1] = k[0] - d;
  for (int i = 2; i <= n; ++i) {
    dp[i][0] = max(dp[i - 1][0], dp[i - 1][1] - d) + t[i - 1];
    dp[i][1] = max(dp[i - 1][1], dp[i - 1][0] - d) + k[i - 1];
  }
  cout << max(dp[n][0], dp[n][1]) << endl;
  return 0;
}