#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
const double epsilon = 1e-7;
long long int ma(long long int x, long long int y) {
  if (x > y) return x;
  return y;
}
long long int x[100000];
long long int v[100000];
long long int fo[100000];
long long int ba[100000];
long long int f2[100000];
long long int b2[100000];
int main(void) {
  int n;
  long long int c;
  scanf("%d %lld", &n, &c);
  long long int vsum = 0;
  for (int i = 0; i < n; i++) {
    scanf("%lld %lld", &x[i], &v[i]);
    vsum += v[i];
    fo[i] = vsum - x[i];
    f2[i] = vsum - x[i] - x[i];
  }
  vsum = 0;
  for (int i = n - 1; i >= 0; i--) {
    vsum += v[i];
    ba[i] = vsum - c + x[i];
    b2[i] = vsum - c - c + x[i] + x[i];
  }
  long long int fm = 0;
  long long int bm = 0;
  int ff = -1;
  int bb = -1;
  for (int i = 0; i < n; i++) {
    if (fm < fo[i] && fo[i] + b2[i + 1] >= fo[i - 1] + b2[i] && b2[i + 1] > 0) {
      fm = fo[i];
      ff = i;
    }
  }
  for (int i = 0; i < n; i++) {
    if (bm < ba[i] && ba[i] + f2[i - 1] >= ba[i + 1] + f2[i] && f2[i - 1] > 0) {
      bm = ba[i];
      bb = i;
    }
  }
  if (fm + bm == 0)
    printf("0\n");
  else {
    long long int f2m = 0;
    long long int b2m = 0;
    for (int i = 0; i < bb; i++) {
      f2m = ma(f2m, f2[i]);
    }
    for (int i = n - 1; i > ff; i--) {
      b2m = ma(b2m, b2[i]);
    }
    printf("%lld\n", ma(fm + b2m, bm + f2m));
  }
  return 0;
}
