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#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<ll, ll> pii;

const int N = 5e5+10;

ll a[N], w[N], mxv[N], n, c;
vector<int> hg[N];

void solve() {
    cin>>n>>c;
    for (int i=1; i<=n; ++i) {
        cin>>a[i]>>w[i];
        hg[a[i]].push_back(w[i]);
    }

    ll ans = 0;

    set<pii> st;
    for (int i=1; i<N; ++i) st.insert({0, i});
    for (int i=N-2; i>=1; --i) {
        vector<pair<pii, pii>> chg;
        for (auto u : hg[i]) {
            pii lst = *st.rbegin();
            pii fnd = *st.find({mxv[u], u});
            assert(fnd.second == u);

            ll nmx = max({mxv[u], lst.first + (lst.second == u ? 0 : -c) + i, fnd.first + i});
            ans = max(ans, nmx);
            chg.push_back({{mxv[u], u}, {nmx, u}});
        }

        for (auto u : chg) {
            st.erase(u.first);
            st.insert(u.second);
            mxv[u.second.second] = u.second.first;
        }
    } cout<<ans<<"\n";
}

int main() {
    int t=1; //cin>>t;
    while (t--) solve();
}