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#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,n) for (int i=(a);i<(n);i++)
#define per(i,a,n) for (int i=(n)-1;i>=(a);i--)
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef vector<int> VI;
typedef basic_string<int> BI;
typedef long long ll;
typedef pair<int,int> PII;
typedef long double db;
mt19937 mrand(1111); 
const ll mod=1000000007;
int rnd(int x) { return mrand() % x;}
ll powmod(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res;}
ll gcd(ll a,ll b) { return b?gcd(b,a%b):a;}
// head

const int N=501000;
int n,c;
ll dpc[N];
VI ht[N];
int main() {
	scanf("%d%d",&n,&c);
	rep(i,1,n+1) {
		int a,w;
		scanf("%d%d",&a,&w);
		ht[a].pb(w);
	}
	ll pc=0;
	rep(i,1,500001) if (!ht[i].empty()) {
		ll dpv=i;
		dpv=max(dpv,dpv+pc-c);
		ll dv=dpv;
		for (auto x:ht[i]) {
			dv=max(dv,dpc[x]+i);
		}
		pc=max(pc,dv);
		vector<pair<int,ll>> w;
		for (auto x:ht[i]) {
			w.pb(mp(x,max(dpc[x]+i,dpv)));
		}
		for (auto [x,d]:w) dpc[x]=max(dpc[x],d);
	}
	printf("%lld\n",pc);
}