#include <bits/stdc++.h>

using namespace std;

#define pb push_back
#define mp make_pair
#define mt make_tuple
#define fi first
#define se second
#define all(a) (a).begin(),(a).end()
#define endl "\n"
#define show(x) cerr << #x << " " << x << endl

typedef long long ll;
typedef unsigned long long ull;

constexpr const int INT_INF=0x3f3f3f3f; // 1061109567
constexpr const ll LL_INF=0x3f3f3f3f3f3f3f3f; // 4557430888798830399

template <typename T> bool chmin(T &a, T b) {
    if (a > b) {a = b;return true;}return false;
}

template <typename T> bool chmax(T &a, T b) {
    if (a < b) {a = b;return true;}return false;
}

// INT
#define GCD(a, b) __gcd(a, b)
template <typename T> T LCM(T a, T b) {return a / GCD(a, b) * b;}
template <typename T> T EXTGCD(T a, T b, T& x, T& y) {
    T d = a;
    if (b != 0) {d=EXTGCD(b,a%b,y,x);y-=(a/b)*x;}
    else x=1,y=0;
    return d;
}
template <typename T> bool is_prime(T a) {
    for(int i=2;i*i<=a;i++)if(a%i==0)return 1;
    return 0;
}

// MOD
const ll MOD = 1000000000 + 7;
#define add(a, b) ((a % MOD) + (b % MOD)) % MOD
#define mul(a, b) ((a % MOD) * (b % MOD)) % MOD
#define sub(a, b) ((a % MOD) + MOD - (b % MOD)) % MOD
template <typename T> T mod_inverse(T a, T mod, bool prime){ // if mod is prime, "prime" is true.
    if(prime){
        T tmp=mod-2,now=a,res=1;while(tmp){if(tmp&1)res=mul(res,now);now=mul(now,now);tmp>>=1;}
        return res;
    }else{T x,y;EXTGCD(a,mod,x,y);return (mod+x%mod)%mod;}
}
#define divide(a, b) ((a % MOD) * (mod_inverse(b, MOD, true))) % MOD

//LLの数値をつかう時は最後にLをつける癖をつけよう
//
//  　　┓　　　┏
//  　＊┗┓　┏┛
//  　　　┫　┣　＊
//  　┏┳┻━┻┳┓
//  　┗┫　　　┣┛
//  ＊　┣　　━┃　＊        Merry Christmas!!
//  　┏┛　　〃┃
//  　┃●　　　┣━┳┓
//  　┗┻┳━━┛　┣┛
//  　＊　┃┏┓┣┓┃
//WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW

int n;
ll c;
ll x[100001];
ll v[100001];
ll LeftV[100001], RightV[100001];

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.precision(10);
    cout << fixed;
#ifdef LOCAL_DEFINE
    FILE *stream1;
    //FILE *stream2;
    stream1 = freopen("/Users/aim_cpo/CLionProjects/competitive/in.txt", "r", stdin);
    //stream2 = freopen("out.txt", "w", stdout);
    if (stream1 == nullptr) return 0;
    //if (stream2 == NULL) return 0;
#endif
    cin >> n >> c;
    for (int i = 0; i < n; i++) {
        cin >> x[i] >> v[i];
    }
    LeftV[0] = v[0];
    for (int i = 1; i < n; i++) {
        LeftV[i] = LeftV[i - 1] + v[i];
    }
    RightV[n - 1] = v[n - 1];
    for (int i = n - 2; i >= 0; i--) {
        RightV[i] = RightV[i + 1] + v[i];
    }
//    for (int i = 0; i < n ; i++) {
//        show(RightV[i]);
//    }
    // left
    ll ans = 0L;
    ll right = 0L, rightd = 0;
    for (int i = n - 1; i >= 0; i--) {
        //show(right);
        //show(rightd);
        ll now = -x[i] + LeftV[i];
        if (now > ans) {
            ans = now;
        }
        if (now - x[i] + right - rightd > ans) {
            ans = now - x[i] + right - rightd;
        }
        if (right - rightd < RightV[i] - c + x[i]) {
            right = RightV[i];
            rightd = c - x[i];
        }
    }
    //show(ans);
    // right
    ll left = 0L, leftd = 0L;
    for (int i = 0; i < n; i++) {
        ll now = -c + x[i] + RightV[i];
        if (now > ans) ans = now;
        if (now - c + x[i] + left - leftd > ans) {
            ans = now - c + x[i] + left - leftd;
        }
        if (left - leftd < LeftV[i] - x[i]) {
            left = LeftV[i];
            leftd = x[i];
        }
    }
    cout << ans << endl;
#ifdef  LOCAL_DEFINE
    cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << "s.\n";
#endif
    return 0;
}

Measured memory_bytes: 3328