//header{{{
//#define _GLIBCXX_DEBUG
#pragma GCC optimize("Ofast")
#include<bits/stdc++.h>
//#include<boost/multiprecision/cpp_int.hpp>
//#include<boost/math/common_factor_rt.hpp>
  
using namespace std;
//using namespace boost::multiprecision;  
//using namespace boost::math;

#define rep(i,n) for(int i=0;i<(n);++i)
#define reps(i,n) for(int i=1;i<=(n);++i)
#define all(x) (x).begin(),(x).end()
#define Fixed fixed << setprecision(12)
#define int int64_t
using pii = pair<int,int>;
constexpr int INF  = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr int mod1 = 1e9+7; 
constexpr int mod2 = 998244353;

template <class A, class B> inline bool chmax(A &a, const B &b) { return b > a && (a = b, true); }
template <class A, class B> inline bool chmin(A &a, const B &b) { return b < a && (a = b, true); }

template <class T> using min_heap = priority_queue<T,vector<T>,greater<T> >;
template <class T> using max_heap = priority_queue<T>;
template <class A, class B> using umap = unordered_map<A,B>;
  
inline int square(int a){ return a * a;}
inline int updiv(int a,int b){ return (a + b - 1) / b; }

constexpr int dx[] = {1,0,-1,0,1,1,-1,-1};
constexpr int dy[] = {0,-1,0,1,1,-1,-1,1};
//}}}

template< typename Monoid >
struct SegmentTree {
    using F = function< Monoid(Monoid, Monoid) >;

    int sz;
    vector< Monoid > seg;

    const F f;
    const Monoid M1;

    SegmentTree(int n, const F f, const Monoid &M1) : f(f), M1(M1) {
        sz = 1;
        while(sz < n) sz <<= 1;
        seg.assign(2 * sz, M1);
    }

    void set(int k, const Monoid &x) {
        seg[k + sz] = x;
    }

    void build() {
        for(int k = sz - 1; k > 0; k--) {
            seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
        }
    }

    void update(int k, const Monoid &x) {
        k += sz;
        seg[k] = x;
        while(k >>= 1) {
            seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
        }
    }

    Monoid query(int a, int b) {
        Monoid L = M1, R = M1;
        for(a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
            if(a & 1) L = f(L, seg[a++]);
            if(b & 1) R = f(seg[--b], R);
        }
        return f(L, R);
    }

    Monoid operator[](const int &k) const {
        return seg[k + sz];
    }

    template< typename C >
    int find_subtree(int a, const C &check, Monoid &M, bool type) {
        while(a < sz) {
            Monoid nxt = type ? f(seg[2 * a + type], M) : f(M, seg[2 * a + type]);
            if(check(nxt)) a = 2 * a + type;
            else M = nxt, a = 2 * a + 1 - type;
        }
        return a - sz;
    }

    template< typename C >
    int find_first(int a, const C &check) {
        Monoid L = M1;
        if(a <= 0) {
            if(check(f(L, seg[1]))) return find_subtree(1, check, L, false);
            return -1;
        }
        int b = sz;
        for(a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
            if(a & 1) {
                Monoid nxt = f(L, seg[a]);
                if(check(nxt)) return find_subtree(a, check, L, false);
                L = nxt;
                ++a;
            }
        }
        return -1;
    }

    template< typename C >
    int find_last(int b, const C &check) {
        Monoid R = M1;
        if(b >= sz) {
            if(check(f(seg[1], R))) return find_subtree(1, check, R, true);
            return -1;
        }
        int a = sz;
        for(b += sz; a < b; a >>= 1, b >>= 1) {
            if(b & 1) {
                Monoid nxt = f(seg[--b], R);
                if(check(nxt)) return find_subtree(b, check, R, true);
                R = nxt;
            }
        }
        return -1;
    }
};


signed main(){
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);

    int n, c;
    cin >> n >> c;

    vector<int> x(n+1), v(n+1);
    SegmentTree<pii> seg(n + 5, [](auto a, auto b){
        if(a.first != b.first){
            return max(a, b);
        }else if(a.second < b.second){
            return a;
        }else{
            return b;
        }
    }, {-LINF, -LINF});

    SegmentTree<int> seg2(n + 5, [](auto a, auto b){ return max(a, b); }, -LINF);

    int cum_v = 0;

    reps(i,n){
        cin >> x[i] >> v[i];
        cum_v += v[i];
        seg.set(i, {cum_v - x[i], i});
        seg2.set(i, cum_v - x[i] * 2);
    }

    seg.set(0, {0, 0});
    seg2.set(0, 0);
    seg.build();
    seg2.build();

    int res = seg.query(0, n+1).first, now = c, sum = 0;
    for(int i = n; i >= 1; --i){
        sum += v[i] - (now - x[i]);
        now = x[i];
        auto [value, pos] = seg.query(0, i);
        res = max({res, sum, value + sum - (c - now), sum + seg2.query(0, i)});
    }

    cout << res << '\n';

    return 0;
}


Measured memory_bytes: 10844