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#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ll long long
#define ld long double
#define ull unsigned long long
#define ff first
#define ss second
#define pii pair<int,int>
#define pll pair<long long, long long>
#define vi vector<int>
#define vl vector<long long>
#define pb push_back
#define rep(i, b) for(int i = 0; i < (b); ++i)
#define rep2(i,a,b) for(int i = a; i <= (b); ++i)
#define rep3(i,a,b,c) for(int i = a; i <= (b); i+=c)
#define count_bits(x) __builtin_popcountll((x))
#define all(x) (x).begin(),(x).end()
#define siz(x) (int)(x).size()
#define forall(it,x) for(auto& it:(x))
using namespace __gnu_pbds;
using namespace std;
typedef tree<int, null_type, less<int>, rb_tree_tag,tree_order_statistics_node_update> ordered_set;
//mt19937 mt;void random_start(){mt.seed(chrono::time_point_cast<chrono::milliseconds>(chrono::high_resolution_clock::now()).time_since_epoch().count());}
//ll los(ll a, ll b) {return a + (mt() % (b-a+1));}
const int INF = 1e9+50;
const ll INF_L = 1e18+40;
const ll MOD = 1e9+7;

const int tree_siz = 1024*1024-1;
ll drzewo[tree_siz];

ll get_max(int akt, int p1, int p2, int s1, int s2)
{
    if(p2 < s1 || p1 > s2) return 0;
    if(p1 >= s1 && p2 <= s2) return drzewo[akt-1];
    return max(get_max(akt*2,p1,(p1+p2)/2,s1,s2),get_max(akt*2+1,(p1+p2)/2+1,p2,s1,s2));
}

void upd(int v)
{
    drzewo[v-1] = max(drzewo[v*2-1],drzewo[v*2]);
    if(v != 1) upd(v/2);
}

void change(int ind, ll x)
{
    drzewo[tree_siz/2+ind] = max(drzewo[tree_siz/2+ind],x);
    upd((tree_siz/2+1+ind)/2);
}

int main()
{
    ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    //random_start();
    int n,c;
    cin >> n >> c;
    vector<pll> new_anses;
    int prev = -1;
    ll ans = 0;
    rep(i,n)
    {
        int a,w;
        cin >> a >> w;
        if(a != prev)
        {
            forall(it,new_anses)
            {
                change(it.ff,it.ss);
            }
            new_anses = {};
        }
        ll new_ans = max(get_max(1,0,tree_siz/2,w,w)+a,max(get_max(1,0,tree_siz/2,0,w-1),get_max(1,0,tree_siz/2,w+1,500'001))+a-c);
        ans = max(ans,new_ans);
        new_anses.pb({w,new_ans});
        prev = a;
    }
    cout << ans <<'\n';
}