#include <bits/stdc++.h>
using namespace std;
#define int long long
#define stoi stoll
using ll=long long;
using vi=vector<int>;
using pii=pair<int,int>;
#define ALL(c) begin(c),end(c)
#define RALL(c) rbegin(c),rend(c)
#define ITR(i,b,e) for(auto i=(b);i!=(e);++i)
#define FORE(x,c) for(auto &x:c)
#define REPF(i,a,n) for(int i=a,i##len=(int)(n);i<i##len;++i)
#define REP(i,n) REPF(i,0,n)
#define REPR(i,n) for(int i=(int)(n);i>=0;--i)
#define SZ(c) ((int)c.size())
#define CONTAIN(c,x) (c.find(x)!=end(c))
#define OUTOFRANGE(y,x,h,w) ((y)<0||(x)<0||(y)>=(h)||(x)>=(w))
#define dump(...)
const signed INF_=1001001001; const long long INF=1001001001001001001LL;
const int DX[9]={0,1,0,-1,1,1,-1,-1,0},DY[9]={-1,0,1,0,-1,1,1,-1,0};
template<class T> ostream& operator<<(ostream &os,const vector<T> &v) {
    ITR(i,begin(v),end(v))os<<*i<<(i==end(v)-1?"":" ");return os;}
template<class T> istream& operator>>(istream &is,vector<T> &v) {
    ITR(i,begin(v),end(v)) is>>*i;return is;}
template<class T,class U> istream& operator>>(istream &is, pair<T,U> &p) {
    is>>p.first>>p.second;return is;}
template<class T> bool chmax(T &a,const T &b){if(a<b){a=b;return 1;}return 0;}
template<class T> bool chmin(T &a,const T &b){if(a>b){a=b;return 1;}return 0;}
template<class T> using heap=priority_queue<T,vector<T>,greater<T>>;
struct{template<class T> operator T(){T x;cin>>x;return x;}} IN;
struct before_main_function {
    before_main_function() {
        #define endl "\n"
        cin.tie(0);ios::sync_with_stdio(false);
        cout<<setprecision(15)<<fixed;
    }
} before_main_function;
//------------------8<------------------------------------8<--------------------

template <class T>
struct SegmentTree {
    using func = function<T(T, T)>;
    int size;
    vector<T> data;
    func merger, updater;
    T ident;
    SegmentTree(int n, func update_func, func merge_func, T identity_element)
     : updater(update_func), merger(merge_func), ident(identity_element) {
        size = 1;
        while (size < n) size <<= 1;
        data.resize(size * 2, ident);
    }
    void update(int i, T x) {
        i += size - 1;
        data[i] = updater(data[i], x);
        while (i > 0) {
            i = (i - 1) / 2;
            data[i] = merger(data[i * 2 + 1], data[i * 2 + 2]);
        }
    }
    T query(int wishL, int wishR, int watchL = 0, int watchR = -1, int k = 0) {
        if(watchR == -1) watchR = size;
        if(watchR <= wishL || wishR <= watchL) return ident;
        if(wishL <= watchL && watchR <= wishR) return data[k];
        T ld = query(wishL, wishR, watchL, (watchL + watchR) / 2, k * 2 + 1);
        T rd = query(wishL, wishR, (watchL + watchR) / 2, watchR, k * 2 + 2);
        return merger(ld, rd);
    }
    T at(int i) {
        return data[i+size-1];
    }
};

signed main() {
    int N, C;
    cin >> N >> C;
    vector<int> x(N), v(N);
    REP(i, N) {
        cin >> x[i] >> v[i];
    }
    int ans = 0, tmp_s = 0, tmp_t = 0;
    vector<int> s(N, 0), t(N, 0);
    REP(i, N) {
        s[i] = (tmp_s += v[i]) - x[i];
        t[i] = (tmp_t += v[N-i-1]) - (C - x[N-i-1]);
    }
    SegmentTree<int> rmq_s(N,[](int old,int new_){return new_;},[](int a,int b){return max(a,b);},0);
    SegmentTree<int> rmq_t(N,[](int old,int new_){return new_;},[](int a,int b){return max(a,b);},0);
    REP(i, N) {
        rmq_s.update(i, s[i]);
        rmq_t.update(i, t[i]);
    }
    chmax(ans, max(rmq_s.query(0, N), rmq_t.query(0, N)));
    tmp_s = 0, tmp_t = 0;
    REP(i, N) {
        tmp_s += v[i];
        tmp_t += v[N - i - 1];
        int S = tmp_s - x[i] * 2 + rmq_t.query(0, N - i - 1);
        int T = tmp_t - (C - x[N - i - 1]) * 2 + rmq_s.query(0, N - i - 1);
        chmax(ans, max(S, T));
    }
    cout << ans << endl;
    return 0;
}



Measured memory_bytes: 7552