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#ifndef LOCAL
#pragma GCC optimize("O3")
#endif
#include <bits/stdc++.h>
#define FOR(i,p,k) for(int i=(p); i<=(k); ++i)
#define REP(i,k) FOR(i,0,(k)-1)
#define RFOR(i,p,n) for(int i=(p); i>=(n); --i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define ssize(x) int((x).size())
#define fi first
#define se second
#define V vector
#define pb push_back
#define eb emplace_back
#define C const
#define pn printf("\n")
using namespace std;
typedef long long ll;
typedef V <int> vi;
typedef V <ll> vll;
typedef C int ci;
typedef C ll cll;
typedef pair <int, int> pii;
typedef pair <ll, ll> pll;
void chmin(auto &a, auto b){a=min(a,b);}
void chmax(auto &a, auto b){a=max(a,b);}
ci inf=2.1e9;
cll infll=4.5e18;
int I(){
    int z;
    scanf("%d", &z);
    //cin>>z;
    return z;
}
void ans(){
    ci n=I(),kara=I();
    ll naj=0;
    ci wcap=5e5;
    vll wzo(wcap+1);
    map <int, vi> mp;
    REP(_, n){
        ci a=I(),w=I();
        mp[a].eb(w);
    }
    for (C auto &[a, vw] : mp){
        V <pair <ll, int>> akt;
        for (ci w : vw){
            cll l=max(naj-kara, wzo[w])+a;
            akt.eb(l, w);
        }
        for (C auto &[l, w] : akt)
            chmax(wzo[w], l),chmax(naj, l);
    }
    printf("%lld\n", naj);
}
int main(){
    //ios_base::sync_with_stdio(0),cin.tie(0);
    int tt=1;
    //tt=I();
    while (tt--)ans();
}