#include <algorithm>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <memory>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>

using namespace std;

#define sz size()
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define all(c) (c).begin(), (c).end()
#define rep(i,a,b) for(int i=(a);i<(b);++i)
#define clr(a, b) memset((a), (b) ,sizeof(a))

#define MOD 1000000007

int dp[105][2];

int main(){
  int n,d;
  cin>>n>>d;
  int t1, k1;
  cin>>t1>>k1;
  clr(dp,0);
  dp[0][0] = t1;
  dp[0][1] = k1-d;
  rep(i,1,n){
    int ti,ki;
    cin>>ti>>ki;
    dp[i][0] = max(dp[i-1][0]+ti,dp[i-1][1]+ti-d);
    dp[i][1] = max(dp[i-1][1]+ki,dp[i-1][0]+ki-d);
  }
  cout << max(dp[n-1][0],dp[n-1][1]) << endl;
  return 0;
}