#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <string>
#include <vector>
#include <iostream>
#include <iomanip>
#include <stdlib.h>

using namespace std;

int main()
{
  int n,d;
  cin >> n >> d;
  int t[101] = {};
  int k[101] = {};
  int tmp1,tmp2;
  cin >> tmp1 >> tmp2;
  t[1] = tmp1;
  k[1] = tmp2 - d;

  for(int i=2;i<=n;i++){
    cin >> tmp1 >> tmp2;
    t[i] = max( t[i-1] + tmp1 , k[i-1] + tmp1 - d );
    k[i] = max( k[i-1] + tmp2 , t[i-1] + tmp2 - d );
  }

  cout << max( t[n] , k[n] ) << endl;

  return 0;
}