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

#define FOR(i,p,q) for(int i=(p); i<=(q); ++i)
#define ROF(i,p,q) for(int i=(p); i>=(q); --i)
#define REP(i,q) for(int i=0; i<(q); ++i)
#define pb push_back
#define as assign
#define rz resize
#define Co const
#define all(X) X.begin(), X.end()
#define rall(X) X.rbegin(), X.rend()
#define sz(X) (int)(X.size())
#define ckmax(a,b) a=max(a,b)
#define ckmin(a,b) a=min(a,b)
#define V vector
typedef long long ll;
typedef mt19937_64 mt;
#ifndef UNCLE
 typedef basic_string<bool> vb;
 typedef basic_string<int> vi;
 typedef basic_string<ll> vl;
#else
 typedef V<bool> vb;
 typedef V<int> vi;
 typedef V<ll> vl;
#endif

constexpr int MX_C=5e5, base=1<<19;
int N;
ll kar;
struct RngTR{
    vl tmx;
    void Bu0(){
        tmx.as(base<<1,0);
    }
    void Upt(int v,ll x){
        v+=base;
        ckmax(tmx[v],x),v/=2;
        while(v) tmx[v]=max(tmx[2*v],tmx[2*v+1]), v/=2;
    }
    ll Mx(int p,int q){
        ll pro=0;
        p+=base-1,q+=base+1;
        while(p/2!=q/2){
            if(!(p&1)) ckmax(pro,tmx[p+1]);
            if(q&1) ckmax(pro,tmx[q-1]);
            p/=2,q/=2;
        }
        return pro;
    }
};
RngTR tr;
struct SS{int h,c; ll rs;};
V<SS> s;

void Input(){
    cin>>N>>kar;
    tr.Bu0(), s.rz(N);
    for(SS &vv:s) cin>>vv.h>>vv.c,--vv.c;
    reverse(all(s));
}
void Solve(){
    int p=0;
    REP(i,N){
        while(p<i&&(s[p].h!=s[i].h)) tr.Upt(s[p].c,s[p].rs),++p;
        s[i].rs=max(tr.Mx(s[i].c,s[i].c),max(tr.Mx(0,s[i].c-1),tr.Mx(s[i].c+1,MX_C-1))-kar)+s[i].h;
    }
    while(p<=N-1) tr.Upt(s[p].c,s[p].rs),++p;
    cout<<tr.Mx(0,MX_C-1)<<"\n";
}
int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0), cout.tie(0);

    Input();
    Solve();
}