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#include<bits/stdc++.h>
 
#define pb push_back
#define pob pop_back
#define eb emplace_back
#define fi first
#define se second
#define turbo ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define mid ((l+r)/2)
#define midLL ((l+r)/2LL)
#define mide ((lo+hi)/2)
#define mideLL ((lo+hi)/2LL)
#define endl '\n'
#define random_shuffle shandom_ruffle
#define lowbit(x) x&(-x)
#define bits(x) __builtin_popcount(x) 
#define mins(se) (*se.begin())
#define maxs(se) (*--se.end())
#define all(x) x.begin(), x.end()
#define log2_floor(i) (i ? __builtin_clzll(1) - __builtin_clzll(i) : -1)
#define siz(cont) ((int)cont.size())
#define each(it, cont) for(auto &it : cont)

using ll = long long;
using pii = std::pair<int, int>;
using pll = std::pair<ll, ll>;
using pli = std::pair<ll, int>;
using pil = std::pair<int, ll>;
using vll = std::vector<pll>;
using vii = std::vector<pii>;
using vi = std::vector<int>;
using vl = std::vector<ll>;
using vvl = std::vector<vl>;
using vvi = std::vector<vi>;
using vvii = std::vector<vii>;
using vli = std::vector<pli>;
using vvli = std::vector<vli>;
using vil = std::vector<pil>;
using vvil = std::vector<vil>;
using vvll = std::vector<vll>;
using vc = std::vector<char>;
using vvc = std::vector<std::vector<char>>;
using vb = std::vector<bool>;
using vvb = std::vector<vb>;

void err() { std::cout << "\n"; fflush(stdout); }
template<class T, class... Ts>
void err(T arg, Ts &... args) {
    std::cout << arg << ' ';
    err(args...);
}

using namespace std;

constexpr int N = 500200;
ll n, c;

set<pll> pri_q;

ll get_best() {
    return (*(--pri_q.end())).fi;
}

signed main() { 
    turbo;
    cin >> n >> c;
    vll pp(n);
    pri_q.insert({0, 0});
    for(auto &[a, w] : pp) {
        cin >> a >> w;
        pri_q.insert({0, w});
    }

    sort(all(pp));
    pp.erase(unique(all(pp)), pp.end());
    reverse(all(pp));

    vvll v;
    v.pb({pp[0]});

    for (int i = 1; i < siz(pp); i++) {
        if (v.back().back().fi == pp[i].fi) {
            v.back().pb(pp[i]);
        }
        else {
            v.pb(vll({pp[i]}));
        }
    }

    vl dp(N);
    vl dp_patch(N);

    for (auto itv : v) {
        for (auto [h, w] : itv) {
            pri_q.erase({dp[w], w});
            ll best = get_best() + h - c;
            best = max(best, dp[w] + h);
            dp_patch[w] = best;
            pri_q.insert({dp[w], w});
        }
        for (auto [h, w] : itv) {
            pri_q.erase({dp[w], w});
            dp[w] = dp_patch[w];
            pri_q.insert({dp[w], w});
        }
    }

    ll best = 0;
    for (int i = 1; i < N; i++)
         best = max(best, dp[i]);
    cout << best;
}