#include <algorithm>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <memory>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>

using namespace std;

#define sz size()
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define all(c) (c).begin(), (c).end()
#define rep(i,a,b) for(int i=(a);i<(b);++i)
#define clr(a, b) memset((a), (b) ,sizeof(a))

#define MOD 1000000007

int main(){
  int n,d;
  cin>>n>>d;
  int t1, k1;
  cin>>t1>>k1;
  int t = t1;
  int k = k1-d;
  rep(i,2,n+1){
    int ti,ki;
    cin>>ti>>ki;
    int t_ = max(t+ti,k+ti-d);
    int k_ = max(k+ki,t+ki-d);
    t = t_;
    k = k_;
  }
  cout << max(t,k) << endl;
  return 0;
}