#include <bits/stdc++.h>
using namespace std;
constexpr int INF = numeric_limits<int>::max() / 2;
constexpr long long LINF = numeric_limits<long long>::max() / 3;
struct Double {
  double d;
  explicit Double(double x) : d(x) {}
};
ostream& operator<<(ostream& os, const Double x) {
  os << fixed << setprecision(20) << x.d;
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& vec) {
  os << "[";
  for (const auto& v : vec) {
    os << v << ",";
  }
  os << "]";
  return os;
}
template <typename T, typename U>
ostream& operator<<(ostream& os, const pair<T, U>& p) {
  os << "(" << p.first << "," << p.second << ")";
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, const set<T>& st) {
  os << "{";
  for (T v : st) os << v << ",";
  os << "}";
  return os;
}
template <typename T, typename U>
inline void chmax(T& x, U y) {
  if (y > x) x = y;
}
template <typename T, typename U>
inline void chmin(T& x, U y) {
  if (y < x) x = y;
}
long long gcd(long long a, long long b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
constexpr double eps = 1e-10;
constexpr long long mod = 1e9 + 7;
const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
long long N, C;
vector<long long> x, v;
long long lsumx[100005], rsumx[100005];
long long lsumv[100005], rsumv[100005];
long long lmax1[100005], rmax1[100005];
long long lmax2[100005], rmax2[100005];
int main() {
  cin >> N >> C;
  x.resize(N);
  v.resize(N);
  for (int i = 0; i < (int)(N); i++) {
    cin >> x[i] >> v[i];
  }
  for (int i = N - 1; i >= 0; i--) {
    lsumx[i] = C - x[i];
    lsumv[i] = lsumv[i + 1] + v[i];
  }
  for (int i = 0; i < N; i++) {
    if (i == 0) {
      rsumx[i] = x[i];
      rsumv[i] = v[i];
    } else {
      rsumx[i] = x[i];
      rsumv[i] = rsumv[i - 1] + v[i];
    }
  }
  for (int i = N - 1; i >= 0; i--) {
    if (i == N - 1) {
      lmax1[i] = max(0ll, lsumv[i] - lsumx[i]);
      lmax2[i] = max(0ll, lsumv[i] - lsumx[i] * 2);
    } else {
      lmax1[i] = max(lmax1[i], lsumv[i] - lsumx[i]);
      lmax2[i] = max(lmax2[i], lsumv[i] - lsumx[i] * 2);
    }
  }
  for (int i = 0; i < N; i++) {
    if (i == 0) {
      rmax1[i] = max(0ll, rsumv[i] - rsumx[i]);
      rmax2[i] = max(0ll, rsumv[i] - rsumx[i] * 2);
    } else {
      rmax1[i] = max(rmax1[i - 1], rsumv[i] - rsumx[i]);
      rmax2[i] = max(rmax2[i - 1], rsumv[i] - rsumx[i] * 2);
    }
  }
  long long ans = 0;
  for (int i = 0; i < N; i++) {
    long long p1 = rmax1[i];
    long long p2 = lmax1[i + 1];
    long long p3 = rmax1[i] + lmax2[i + 1];
    long long p4 = rmax2[i] + lmax1[i + 1];
    long long mx = max({p1, p2, p3, p4});
    ans = max(mx, ans);
  }
  cout << ans << endl;
}
