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

#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define repi(i,a,b) for(int i=int(a);i<(int)(b);i++)
#define all(x) (x).begin(),(x).end()
#define foreach(u,v) for(auto (u) : (v))
#define pb push_back
#define mp make_pair
#define mt make_tuple

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<ll, ll> pll;
typedef vector<ll> vl;

const int inf = 1e9;
const ll linf = 1e18;
const ll mod = 1e9 + 7;
const double eps = 1e-9;

int main()
{
  int n, d;
  cin >> n >> d;

  vvi t(2, vi(n));
  rep(i, n) cin >> t[0][i] >> t[1][i];
  t[1][0] -= d;

  vvi dp(n+1, vi(2, 0));
  rep(i, n){
    rep(j, 2){
      rep(k, 2){
        dp[i+1][k] = max(dp[i+1][k], dp[i][j]+t[k][i]-d*(j%2!=k%2));
      }
    }
  }

  cout << max(dp[n][0], dp[n][1]) << endl;

/*
  rep(i, n+1){
    rep(j, 2){
      cout << dp[i][j] << " ";
    }
    cout << endl;
  }
  */

  return 0;
}