#include <iostream>
#include <stdio.h>
#include <string>
#include <vector>
#include <set>
#include <cmath>
#include <algorithm>
#define MP make_pair
using ll = long long;
using namespace std;

int main()
{
  ll N, D;
  cin >> N >> D;
  vector<ll>T(N);
  vector<ll>K(N);
  for(int i=0; i < N; i++){
    cin >> T[i];
    cin >> K[i];
  }
  vector<vector<ll> >dp(N, vector<ll>(2));
  dp[0][0] = T[0];
  dp[0][1] = K[0]-D;
  for(int i=1; i < N; i++){
    dp[i][0] = max(dp[i-1][0]+T[i], dp[i-1][1]-D+T[i]);
    dp[i][1] = max(dp[i-1][0]-D+K[i], dp[i-1][1]+K[i]);
  }
  cout << max(dp[N-1][0], dp[N-1][1]) << endl;
  return 0;
}