/* -*- coding: utf-8 -*-
 *
 * 561.cc: No.561 東京と京都 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 100;
const int INF = 1 << 30;

/* typedef */

/* global variables */

int es[MAX_N][2];
int dp[MAX_N + 1][2];

/* subroutines */

inline void setmax(int &a, int b) { if (a < b) a = b; }

/* main */

int main() {
  int n, d;
  scanf("%d%d", &n, &d);

  for (int i = 0; i < n; i++) scanf("%d%d", &es[i][0], &es[i][1]);
  for (int i = 0; i <= n; i++) dp[i][0] = dp[i][1] = -INF;
  
  dp[0][0] = 0;

  for (int i = 0; i < n; i++)
    for (int j = 0, k = 1; j < 2; j++, k--) {
      setmax(dp[i + 1][j], dp[i][j] + es[i][j]);
      setmax(dp[i + 1][k], dp[i][j] + es[i][k] - d);
    }

  printf("%d\n", max(dp[n][0], dp[n][1]));
  return 0;
}