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#include<bits/stdc++.h>

using namespace std;

#define ll long long

const ll MAXN = 5e5 + 7;
const ll base = 1 << 19;
const ll wmax = 5e5;

vector<ll> tab[MAXN];
stack<pair<ll, ll>> stosik;
ll tree[base*2];

void push(ll cel, ll val){
    cel += base-1;
    tree[cel] = max(tree[cel], val);
    while(cel > 0){
        cel /= 2;
        tree[cel] = max(tree[cel*2], tree[cel*2+1]);
    }
}

ll query(ll w, ll pocz, ll kon, ll l, ll r){
    if(r < l){
        return -1;
    }
    if(r < pocz || kon < l){
        return -1;
    }
    else if(pocz <= l && r <= kon){
        return tree[w];
    }
    else{
        ll mid = (l+r)/2;
        ll left = w*2;
        ll right = w*2+1;
        return max(query(left, pocz, kon, l, mid), query(right, pocz, kon, mid+1, r));
    }
}

int32_t main(){
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    ll n;
    ll c;
    cin >> n >> c;

    for(ll i = 1; i <= n ; i++){
        ll a, w;
        cin >> a >> w;
        tab[a].push_back(w);
    }

    for(ll i = 1; i <= wmax ; i++){
        for(ll w : tab[i]){
            ll dp = max(query(1, 1, w-1, 1, base), query(1, w+1, wmax, 1, base))-c;
            dp = max(dp, query(1, w, w, 1, base));
            dp += i;
            stosik.push({w, dp});
        }
        while(!stosik.empty()){
            pair<ll, ll> para = stosik.top();
            stosik.pop();
            push(para.first, para.second);
        }
    }

    cout << query(1, 1, wmax, 1, base) << "\n";

    return 0;
}