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#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define rep(a, b) for(int a = 0; a < (b); ++a)
#define st first
#define nd second
#define pb push_back
#define all(a) a.begin(), a.end()
int main() {
	ios_base::sync_with_stdio(0); cin.tie(0);
	ll n, c;
	cin >> n >> c;
	vector<pair<ll,ll>>T(n);
	rep(i, n) cin >> T[i].st >> T[i].nd;
	sort(all(T));
	vector<pair<ll,ll>>P;
	P.pb(T[0]);
	rep(i, n-1) if(T[i]!=T[i+1]) P.pb(T[i+1]);
	map<ll,ll>dpkol;
	ll dpbez=0;
	vector<ll>dp(P.size());
	int j=0;
	n=P.size();
	rep(i, n) {
		if(i && P[i].st!=P[i-1].st) {
			while(j<i) {
				dpkol[P[j].nd]=max(dpkol[P[j].nd], dp[j]);
				dpbez=max(dpbez, dp[j]);
				++j;
			}
		}
		dp[i]=max(dpkol[P[i].nd], dpbez-c)+P[i].st;
	}
	ll ans=0;
	for(auto i : dp) ans=max(ans, i);
	cout << ans << '\n';
}