#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, c;
  cin >> n >> c;
  long long x[n], v[n], dp[n + 1], result = 0;
  dp[0] = 0;
  for (long long i = 0; cin >> x[i] >> v[i]; i++) {
  }
  for (long long i = 0, sum = 0; i < n; i++) {
    sum += v[i];
    dp[i + 1] = max(dp[i], sum - x[i]);
  }
  for (long long i = n - 1, sum = 0; i > 0; i--) {
    sum += v[i];
    result = max({result, sum - 2 * (c - x[i]) + dp[i], dp[i]});
  }
  dp[n] = 0;
  for (long long i = n - 1, sum = 0; i >= 0; i--) {
    sum += v[i];
    dp[i] = max(dp[i + 1], sum - c + x[i]);
  }
  for (long long i = 0, sum = 0; i < n; i++) {
    sum += v[i];
    result = max({result, sum - 2 * x[i] + dp[i + 1], dp[i + 1]});
  }
  cout << result;
  return 0;
}
