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#include <bits/stdc++.h>
//#define debug(x) cout<<#x<<" = "<<x<<"\n"
#define debug(x) x
#define pb push_back
#define ins insert
#define fi first
#define se second
 
using namespace std;
using ull = unsigned long long;
using ll = long long;
using ld = long double;
 
template <typename H, typename T> 
ostream& operator<<(ostream& os, pair<H, T> m){
	return os <<"("<< m.fi<<", "<<m.se<<")";
}
 
template <typename H> 
ostream& operator<<(ostream& os, vector<H> V){
	os<<"{";
	for(int i=0; i<V.size(); i++){
		if(i)os<<" ";
		os<<V[i];
	}
	os<<"}";
	return os;
}
 
void solve();
  


int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);

	int t = 1;
	//cin>>t;
	while(t--) {

		solve();
	}


	return 0;
}

map<ll,ll> best;
ll MEGABEST = 0; //MEGAWONSZ 9000
ll MEGABEST2 = 0;


void solve(){
	ll n,c;
	ll last = -1;
	cin>>n>>c;
	queue<pair<ll,ll> > Q;
	for(int i = 0; i < n; i++){
		ll a,w;
		cin>>a>>w;
		if(a != last){
			MEGABEST = max(MEGABEST,MEGABEST2);
			while(!Q.empty()){
				pair<ll,ll> p = Q.front();
				best[p.fi] = max(best[p.fi], p.se);
				Q.pop();
			}
		}
		if(best.find(w) == best.end()){
			
			Q.push({w,max(MEGABEST + a -  c, a)});
			MEGABEST2 = max(MEGABEST2 , max(MEGABEST + a -  c, a));
		}else{
			Q.push({w,max(max(a, MEGABEST + a -c),best[w] + a)});
			MEGABEST2 = max(MEGABEST2 , max(max(a, MEGABEST + a -c),best[w] + a));
		}
		last = a;
	}
	cout<<max(MEGABEST2,MEGABEST)<<"\n";
	



}