
public class Main {
  private static void solve() {
    int n = ni();
    long c = nl();
    long[][] sushi = new long[n][2];
    for (int i = 0; i < n; i ++) {
      sushi[i][0] = nl();
      sushi[i][1] = nl();
    }
    
    long retA = f(sushi, c);

    for (int i = 0, j = n - 1; i < j; i ++, j --) {
      long[] tmp = sushi[i];
      sushi[i] = sushi[j];
      sushi[j] = tmp;
    }
    for (int i = 0; i < n;i  ++) {
      sushi[i][0] = c - sushi[i][0];
    }
    
    long retB = f(sushi, c);
    System.out.println(Math.max(retA, retB));
  }
  
  private static long f(long[][] sushi, long c) {
    int n = sushi.length;

    long[][] dp = new long[n + 1][2];
    long now = 0;
    long pos = 0;
    for (int i = 0; i < n; i ++) {
      now += sushi[i][1] - (sushi[i][0] - pos);
      pos = sushi[i][0];
      if (now > dp[i][1]) {
        dp[i + 1][1] = now;
        dp[i + 1][0] = i;
      } else {
        dp[i + 1][1] = dp[i][1];
        dp[i + 1][0] = dp[i][0];
      }
    }
    
    now = 0;
    long ret = dp[n][1];
    long x = 0;
    for (int i = n - 1; i >= 1; i --) {
      x += sushi[i][1];
      long cost = (c - sushi[i][0]) * 2;
      long ben = x + dp[i][1];
      ret = Math.max(ret, ben - cost);
    }
    return ret;
  }

  public static void main(String[] args) {
    new Thread(null, new Runnable() {
      @Override
      public void run() {
        long start = System.currentTimeMillis();
        String debug = args.length > 0 ? args[0] : null;
        if (debug != null) {
          try {
            is = java.nio.file.Files.newInputStream(java.nio.file.Paths.get(debug));
          } catch (Exception e) {
            throw new RuntimeException(e);
          }
        }
        reader = new java.io.BufferedReader(new java.io.InputStreamReader(is), 32768);
        solve();
        out.flush();
        tr((System.currentTimeMillis() - start) + "ms");
      }
    }, "", 64000000).start();
  }

  private static java.io.InputStream is = System.in;
  private static java.io.PrintWriter out = new java.io.PrintWriter(System.out);
  private static java.util.StringTokenizer tokenizer = null;
  private static java.io.BufferedReader reader;

  public static String next() {
    while (tokenizer == null || !tokenizer.hasMoreTokens()) {
      try {
        tokenizer = new java.util.StringTokenizer(reader.readLine());
      } catch (Exception e) {
        throw new RuntimeException(e);
      }
    }
    return tokenizer.nextToken();
  }

  private static double nd() {
    return Double.parseDouble(next());
  }

  private static long nl() {
    return Long.parseLong(next());
  }

  private static int[] na(int n) {
    int[] a = new int[n];
    for (int i = 0; i < n; i++)
      a[i] = ni();
    return a;
  }

  private static char[] ns() {
    return next().toCharArray();
  }

  private static long[] nal(int n) {
    long[] a = new long[n];
    for (int i = 0; i < n; i++)
      a[i] = nl();
    return a;
  }

  private static int[][] ntable(int n, int m) {
    int[][] table = new int[n][m];
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < m; j++) {
        table[i][j] = ni();
      }
    }
    return table;
  }

  private static int[][] nlist(int n, int m) {
    int[][] table = new int[m][n];
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < m; j++) {
        table[j][i] = ni();
      }
    }
    return table;
  }

  private static int ni() {
    return Integer.parseInt(next());
  }

  private static void tr(Object... o) {
    if (is != System.in)
      System.out.println(java.util.Arrays.deepToString(o));
  }
}


