#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <utility>
#include <tuple>
#include <cstdint>
#include <cstdio>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <deque>
#include <unordered_map>
#include <unordered_set>
#include <bitset>
#include <cctype>
#include <functional>
#include <ctime>
#include <cmath>
#include <limits>
#include <numeric>
#include <type_traits>
#include <iomanip>
#include <float.h>
#include <math.h>
using namespace std;
using ll = long long;


unsigned euclidean_gcd(unsigned a, unsigned b) {
    if (a < b) return euclidean_gcd(b, a);
    unsigned r;
    while ((r = a % b)) {
        a = b;
        b = r;
    }
    return b;
}

class UnionFind {
public:
    vector <ll> par;
    vector <ll> siz;
    UnionFind(ll sz_) : par(sz_), siz(sz_, 1LL) {
        for (ll i = 0; i < sz_; ++i) par[i] = i;
    }
    void init(ll sz_) {
        par.resize(sz_);
        siz.assign(sz_, 1LL);
        for (ll i = 0; i < sz_; ++i) par[i] = i;
    }
    ll root(ll x) {
        while (par[x] != x) {
            x = par[x] = par[par[x]];
        }
        return x;
    }
    bool merge(ll x, ll y) {
        x = root(x);
        y = root(y);
        if (x == y) return false;
        if (siz[x] < siz[y]) swap(x, y);
        siz[x] += siz[y];
        par[y] = x;
        return true;
    }

    bool issame(ll x, ll y) {
        return root(x) == root(y);
    }

    ll size(ll x) {
        return siz[root(x)];
    }
};

long long modpow(long long a, long long n, long long mod) {
    long long res = 1;
    while (n > 0) {
        if (n & 1) res = res * a % mod;
        a = a * a % mod;
        n >>= 1;
    }
    return res;
}

long long modinv(long long a, long long mod) {
    return modpow(a, mod - 2, mod);
}


int main() {
    ll n, c;
    cin >> n >> c;
    vector<pair<ll, ll>> z(n+2);
    for (int i = 1; i < n+1; i++) {
        cin >> z.at(i).first >> z.at(i).second;
    }
    z.at(n + 1).first = c;
    vector<ll> juntan(n+1);
    ll zansjuntan = 0;
    for (int i = 1; i < n+1; i++) {
        zansjuntan += z.at(i - 1).first - z.at(i).first + z.at(i).second;
        juntan.at(i) = max(juntan.at(i - 1), zansjuntan);
    }
    vector<ll> junhuk(n + 1);
    ll zansjunhuk = 0;
    for (int i = 1; i < n + 1; i++) {
        zansjunhuk += (z.at(i - 1).first - z.at(i).first)* 2 + z.at(i).second;
        junhuk.at(i) = max(junhuk.at(i - 1), zansjunhuk);
    }
    reverse(z.begin(), z.end());
    vector<ll> yjuntan(n + 1);
    ll yzansjuntan = 0;
    for (int i = 1; i < n + 1; i++) {
        yzansjuntan += (z.at(i - 1).first - z.at(i).first)* -1 + z.at(i).second;
        yjuntan.at(i) = max(yjuntan.at(i - 1),yzansjuntan);
    }
    vector<ll> yjunhuk(n + 1);
    ll yzansjunhuk = 0;
    for (int i = 1; i < n + 1; i++) {
        yzansjunhuk += (z.at(i - 1).first - z.at(i).first) * -2 + z.at(i).second;
        yjunhuk.at(i) = max(yjunhuk.at(i - 1), yzansjunhuk);
    }
    reverse(yjuntan.begin(), yjuntan.end());
    reverse(yjunhuk.begin(), yjunhuk.end());
    ll ans = 0;
    for (int i = 0; i < n + 1; i++) {
        ans = max(juntan.at(i) + yjunhuk.at(i), ans);
        ans = max(yjuntan.at(i) + junhuk.at(i), ans);
    }
    cout << ans << endl;
}

Measured memory_bytes: 4992