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

int n, d;
int a[101], b[101];
int dp[101][2];

int solver(int v = -1, bool f = false) {
  if (v == n) return 0;

  int &res = dp[v][f];
  if (v != -1 && ~res) return res;

  res = max(solver(v + 1, false) + a[v + 1] - (f ? d : 0),
            solver(v + 1, true) + b[v + 1] - (f ? 0 : d));

  return res;
}

int main() {
  cin >> n >> d;
  for (int i = 0; i < n; ++i) {
    cin >> a[i] >> b[i];
  }

  memset(dp, -1, sizeof(dp));
  cout << solver() << endl;
}