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// {{{ file: wie.cpp | time: 09:59 14.03.2025
#define _USE_MATH_DEFINES
#include <bits/stdc++.h>

#define each(...)    for (auto& __VA_ARGS__)
#define rep(i, b, e) for (int i = (b); i <= (e); i++)
#define rev(i, b, e) for (int i = (e); i >= (b); i--)
#define mp make_pair
#define mt make_tuple
#define x first
#define y second
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define endl '\n'
#define tC template<class
#define $(x) #x<<'='<<(x)<<' '

using namespace std;
using ll = long long;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using vb = vector<bool>;
using vi = vector<int>;
using vl = vector<ll>;
using vs = vector<string>;
using vpi = vector<pii>;
using vpl = vector<pll>;
using vvi = vector<vi>;
using vvl = vector<vl>;
tC T, class C=greater<T>> using min_priority_queue = priority_queue<T,vector<T>,C>;
tC T> int sz(const T& a) { return (int)a.size(); }
tC T> bool amin(T& a, T b) { return b < a ? a = b, 1 : 0; }
tC T> bool amax(T& a, T b) { return b > a ? a = b, 1 : 0; }
const int oo = 1e9+1;
const ll OO = ll(1e18)+1;

auto now() { return chrono::high_resolution_clock::now().time_since_epoch().count(); }
mt19937 rnd(4488);
tC T> T rand(T lo, T hi) { return uniform_int_distribution<T>{lo,hi}(rnd); }

struct Debug {
#ifdef SPONGE
  tC T>Debug operator<<(const T& x) { cerr<<"\033[1;33m"<<x<<"\033[0m"; return *this; }
#else
  tC T>Debug operator<<(const T&) { return *this; }
#endif
} dbg;

namespace { void solve(); }
// }}}

int main()
{
  //freopen("input.txt","r",stdin);
  //freopen("output.txt","w",stdout);
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.precision(15);
  cout.setf(ios::fixed,ios::floatfield);
  cerr.precision(15);
  cerr.setf(ios::fixed,ios::floatfield);
  //rnd.seed(now());
  int t=1;
  //cin>>t;
  rep(i,1,t){
    //cout<<"Case #"<<i<<": ";
    solve();
  }
}

namespace {

const int N=1<<19;
const int maks=500000;
ll drze[2*N],dp[N];
vi grupy[N];

ll pytaj(int p,int k)
{
  p+=N;
  k+=N;
  ll ret=0;
  while(p<=k){
    if(p%2==1) amax(ret,drze[p]);
    if(k%2==0) amax(ret,drze[k]);
    p=(p+1)/2;
    k=(k-1)/2;
  }
  return ret;
}

void aktu(int i,ll x)
{
  i+=N;
  amax(drze[i],x);
  while(i>1){
    i/=2;
    drze[i]=max(drze[2*i],drze[2*i+1]);
  }
}

void solve()
{
  int n,c;
  cin>>n>>c;

  rep(i,1,n){
    int a,w;
    cin>>a>>w;
    grupy[a].pb(w);
  }

  rep(a,1,maks){
    each(w:grupy[a]) dp[w]=0;
    each(w:grupy[a]){
      amax(dp[w],a+pytaj(1,w-1)-c);
      amax(dp[w],a+pytaj(w+1,maks)-c);
      amax(dp[w],a+pytaj(w,w));
    }
    each(w:grupy[a]) aktu(w,dp[w]);
  }

  cout<<pytaj(1,maks)<<endl;
}

}