#include <bits/stdc++.h>
const int LG = 21;
const int FN = 400005;
const long long MOD = 1e9 + 7;
const long long INF = 1e9;
const long long INFLL = 1e18;
using namespace std;
int cx[4] = {-1, 0, 1, 0};
int cy[4] = {0, -1, 0, 1};
long long inq(long long x, long long y) {
  if (!y) return 1 % MOD;
  long long l = inq(x, y / 2);
  if (y % 2) return l * l % MOD * x % MOD;
  return l * l % MOD;
}
long long rev(long long x) { return inq(x, MOD - 2); }
bool __precomputed_combinatorics = 0;
vector<long long> __fact, __ufact, __rev;
void __precompute_combinatorics() {
  __precomputed_combinatorics = 1;
  __fact.resize(FN);
  __ufact.resize(FN);
  __rev.resize(FN);
  __rev[1] = 1;
  for (int i = 2; i < FN; i++)
    __rev[i] = MOD - __rev[MOD % i] * (MOD / i) % MOD;
  __fact[0] = 1, __ufact[0] = 1;
  for (int i = 1; i < FN; i++)
    __fact[i] = __fact[i - 1] * i % MOD,
    __ufact[i] = __ufact[i - 1] * __rev[i] % MOD;
}
long long fact(int x) {
  if (!__precomputed_combinatorics) __precompute_combinatorics();
  return __fact[x];
}
long long cnk(int n, int k) {
  if (!__precomputed_combinatorics) __precompute_combinatorics();
  return __fact[n] * __ufact[n - k] % MOD * __ufact[k] % MOD;
}
const int N = 100500;
int n;
long long L;
long long x[N], v[N];
long long pv[N], sv[N];
long long prefbest1[N];
long long prefbest2[N];
long long suffbest1[N];
long long suffbest2[N];
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> L;
  for (int(i) = 0; (i) != (n); (i)++) cin >> x[i] >> v[i];
  long long ans = 0;
  pv[0] = v[0];
  for (int i = 1; i < n; i++) pv[i] = pv[i - 1] + v[i];
  sv[n - 1] = v[n - 1];
  for (int i = n - 2; i >= 0; i--) sv[i] = sv[i + 1] + v[i];
  for (int i = 0; i < n; i++) {
    prefbest1[i] = pv[i] - x[i];
    prefbest2[i] = pv[i] - 2 * x[i];
    if (i) prefbest1[i] = max(prefbest1[i], prefbest1[i - 1]);
    if (i) prefbest2[i] = max(prefbest2[i], prefbest2[i - 1]);
  }
  for (int i = 0; i < n; i++) {
    suffbest1[i] = sv[i] - (L - x[i]);
    suffbest2[i] = sv[i] - 2 * (L - x[i]);
    if (i) suffbest1[i] = max(suffbest1[i], suffbest1[i + 1]);
    if (i) suffbest2[i] = max(suffbest2[i], suffbest2[i + 1]);
  }
  ans = max(ans, pv[n - 1] - x[n - 1]);
  ans = max(ans, sv[0] - (L - x[0]));
  for (int i = 0; i + 1 < n; i++) {
    ans = max(ans, prefbest1[i] + suffbest2[i + 1]);
    ans = max(ans, prefbest2[i] + suffbest1[i + 1]);
  }
  cout << ans;
}
