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#include "bits/stdc++.h"
using namespace std;
#define rep(i,a,b) for(int i=(a); i<(b); ++i)
#define all(x) x.begin(),x.end()
#define sz(x) int(x.size())
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<vi> vvi;

const int N = 500010;
ll scores[N];

int main(){
    cin.tie(NULL),cin.sync_with_stdio(false);
    
    ll n,c; cin >> n >> c;
    vector<array<ll,2>> a(n);
    for(auto& [x,y] : a){
        cin >> x >> y;
    }

    sort(all(a),[&](auto p, auto q){
        if (p[0] != q[0]) return p[0] > q[0];
        return p[1] < q[1];
    });

    a.erase(unique(all(a)),end(a)); // each color at most one size 
    n = sz(a);

    ll mx = 0;
    ll nxtmx = 0;
    rep(i,0,n){
        if (i and a[i][0] != a[i-1][0]){
            mx = max(mx, nxtmx);
        }

        // start with block
        ll score = a[i][0];
        // start with best prev
        score = max(score, a[i][0] + scores[a[i][1]]);
        // start with arbitrary prev
        score = max(score, mx + a[i][0] - c);

        scores[a[i][1]] = max(scores[a[i][1]], score);
        nxtmx = max(nxtmx, score);
    }

    mx = max(mx,nxtmx);

    cout << mx << '\n';
}