#include <bits/stdc++.h>
#define range(i, l, r) for(long long int (i) = (l); (i) < (r); (i)++)
#define reversed_range(i, l, r) for (long long int (i) = (r) - 1; (i) >= l; (i)--)
using namespace std;
template <typename T>
using vec = vector<T>;
using lint = long long;
using ulint = unsigned long long;
using pint = pair<int, int>;
using plint = pair<lint, lint>;

template <typename S, typename T>
ostream& operator <<(ostream& os, pair<S, T> p) {
    os << "(";
    os << p.first << ", " << p.second;
    return os << ")";
}

template <typename T>
ostream& operator <<(ostream& os, vec<T> v) {
    os << "[";
    if (v.size() == 0) return os << "]";
    for (int i = 0; i < v.size() - 1; i++) {
        os << v.at(i) << ", ";
    }
    return os << v.at(v.size() - 1) << "]";
}

template <typename T>
ostream& operator <<(ostream& os, set<T>& s) {
    os << "{";
    if (s.begin() == s.end()) return os << "}";
    auto it_first_item = s.begin();
    os << *it_first_item;
    for (auto it = ++it_first_item; it != s.end(); it++) {
        os << ", " << *it;
    }
    return os << "}";
}

template <typename T>
ostream& operator <<(ostream& os, unordered_set<T>& s) {
    os << "{";
    if (s.begin() == s.end()) return os << "}";
    auto it_first_item = s.begin();
    os << *it_first_item;
    for (auto it = ++it_first_item; it != s.end(); it++) {
        os << ", " << *it;
    }
    return os << "}";
}

template <typename K, typename V>
ostream& operator <<(ostream& os, map<K, V> m) {
    os << "{";
    if (m.begin() == m.end()) return os << "}";
    auto it_first_item = m.begin();
    os << it_first_item->first << ": " << it_first_item->second;
    for (auto it = ++it_first_item; it != m.end(); it++) {
        os << ", " << it->first << ": " << it->second;
    }
    return os << "}";
}

template <typename K, typename V>
ostream& operator <<(ostream& os, unordered_map<K, V> m) {
    os << "{";
    if (m.begin() == m.end()) return os << "}";
    auto it_first_item = m.begin();
    os << it_first_item->first << ": " << it_first_item->second;
    for (auto it = ++it_first_item; it != m.end(); it++) {
        os << ", " << it->first << ": " << it->second;
    }
    return os << "}";
}

lint pow(lint num, lint e, lint MOD) {
    lint res = 1;
    lint cur_num = num;
    while (e) {
        if (e & 1) {
            res *= cur_num;
            res %= MOD;
        }
        cur_num *= cur_num;
        cur_num %= MOD;
        e >>= 1;
    }
    return res;
}

int main() {
    cin.tie(0); cout.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    lint N, C; cin >> N >> C;
    vec<plint> sushi_list(N);
    range(i, 0, N) {
        lint x, v; cin >> x >> v;
        sushi_list.at(i) = {x, v};
    }
    auto max_until = [](const vec<lint>& bit, lint idx) {
        lint result = -1LL << 58;
        while (idx > 0) {
            result = max(result, bit.at(idx));
            idx -= idx & -idx;
        }
        return result;
    };
    auto update = [](vec<lint>& bit, lint idx, lint val) {
        while (idx <= bit.size()) {
            bit.at(idx) = max(bit.at(idx), val);
            idx += idx & -idx;
        }
    };
    vec<lint> pref_energy(4 * N + 10, -1LL << 58), pref_energy_double_dist(4 * N + 10, -1LL << 58);
    lint cur_val = 0;
    range(i, 0, N) {
        cur_val += sushi_list.at(i).second;
        lint cur_energy = cur_val - sushi_list.at(i).first;
        lint cur_energy_double_dist = cur_val - sushi_list.at(i).first * 2;
        update(pref_energy, i + 1, cur_energy);
        update(pref_energy_double_dist, i + 1, cur_energy_double_dist);
    }
    lint res = max(max_until(pref_energy, N), 0LL);
    lint right_val = 0;
    reversed_range(i, 1, N) {
        lint right_dist = C - sushi_list.at(i).first;
        right_val += sushi_list.at(i).second;
        lint energy = max(max_until(pref_energy, i), 0LL) + right_val - 2 * right_dist;
        lint energy_dd = max(max_until(pref_energy_double_dist, i), 0LL) + right_val - right_dist;
        res = max({res, energy, energy_dd});
    }
    lint right_dist = C - sushi_list.at(0).first;
    right_val += sushi_list.at(0).second;
    lint energy = right_val - right_dist;
    res = max(res, energy);
    cout << res << "\n";
}

Measured memory_bytes: 8064