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#include <bits/stdc++.h>

using namespace std;

#define st first
#define nd second
#define pb push_back
#define rep(i,a,b) for(int i = a; i <= b; i++)
#define irep(i,a,b) for(int i = a; i >= b; i--)
typedef long long ll;
typedef long double ld;
//typedef __int128 int128;
typedef vector<int> vi;
typedef pair<int,int> pi; 
typedef pair<double,double> pd;
typedef pair<ll,ll> pl;

const int max_n = 5e5+7;

ll dp[max_n];
set<ll> arr[max_n];

int main(){

    ios::sync_with_stdio(0);
    cin.tie(0);

    ll n,c; cin >> n >> c;
    rep(i,1,n){
        ll a,w; cin >> a >> w;
        arr[a].insert(w);
    }

    ll m = 0;
    for(ll i = max_n-1; i >= 1; i--){
        if(arr[i].empty()) continue;
        ll new_m = 0;
        for(auto x:arr[i]){
            ll val = max(m-c,dp[x]); val += i;
            new_m = max(new_m,val);
            dp[x] = val;
        }
        m = max(m,new_m);
    }
    cout << m;

    return 0;
}

//g++ -O3 -static -Wall .cpp -std=c++17