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#include<bits/stdc++.h>
#define FOR(i,a,b) for(int i=a;i<b;++i)
#define FORD(i,a,b) for(int i=a;i>=b;--i)
#define PB push_back
#define EB emplace_back
#define FI first
#define SE second
#define umap unordered_map
#define uset unordered_set
#define vi vector<int>
#define vvi vector<vi>
#define vll vector<ll>
#define vvll vector<vll>
#define vpii vector<pii>
#define pii pair<ll, ll>
#define pll pair<ll, ll>
#define ALL(X) (X).begin(),(X).end()
#ifndef DEBUG
#define endl (char)10
#endif
using namespace std;
using ll = long long;
using ld = long double;

template <class T>
ostream& operator<< (ostream& os, vector<T>& vec){
    for(auto& t : vec) os << t << " ";
    return os;
}
template<class T, class U>
ostream& operator<< (ostream& os, const pair<T, U>& p){
    os << p.FI << " " << p.SE;
    return os;
}
template<class T, class U>
istream& operator>> (istream& is, pair<T, U>& p){
    is >> p.FI >> p.SE;
    return is;
}
template <class T>
istream& operator>> (istream& is, vector<T>& vec){
    for(auto& p : vec) is >> p;
    return is;
}

int main () {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    
    int n, c;
    cin >> n >> c;
    vpii K(n);
    cin >> K;
    reverse(ALL(K));
    vll best(500001, LLONG_MIN);
    vll dp(n);
    ll best_dp = LLONG_MIN + c + 1;
    int b = 0;
    while(b < n){
		int e = b + 1;
		while(e < n && K[e].FI == K[b].FI) e++;
		FOR(i,b,e) dp[i] = max({best_dp - c, 0LL, best[K[i].SE]}) + K[i].FI;
		FOR(i,b,e) {
			best[K[i].SE] = max(best[K[i].SE], dp[i]);
			best_dp = max(best_dp, dp[i]);
		}
		b = e;
	}
	cout << *max_element(ALL(dp)) << endl;
    
}