#include <bits/stdc++.h>

using namespace std;

#define scnaf scanf
#define sacnf scanf
#define sancf scanf

#define ln '\n'
#define y0 y3487465
#define y1 y8687969
#define j0 j1347829
#define j1 j234892
#define PERM(v) next_permutation(all(v))
#define UNIQUE(v) sort(all(v));(v).erase(unique(all(v)), v.end())
#define CIN(type,x) type x;cin >> x
#define MP(a,b) make_pair((a),(b))
#define PB(a) push_back((a))
#define P(p) cout << (p) << endl;
#define S(s) cin >> (s);
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define chmax(x,y) x=max(x,y)
#define chmin(x,y) x=min(x,y)
#define tmax(x,y,z) max(x,max(y,z))
#define tmin(x,y,z) min(x,min(y,z))
#define fastIO ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0)

#define reps(i,a,b) for(int (i)=(a); (i)<(b); ++(i))
#define rep(i,n) reps(i,0,(n))
#define rreps(i,a,b) for(int (i)=(a); (i)>=(b); --(i))
#define rrep(i,n) reprt(i,n,0)

typedef long double ld;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<string> vst;
typedef vector<bool> vb;
typedef vector<ld> vld;
typedef vector<pii> vpii;
typedef vector<pll> vpll;
typedef vector<vector<int> > mat;

const int MOD = 1e9 + 7;
const int INF = 1e9;
const ll INFF = 1e18;
const double EPS = 1e-9;
const double PI  = acos(-1.0);

ll gcd(ll a,ll b) {return b?gcd(b,a%b):a;}
ll lcm(ll a,ll b) {return a*b/gcd(a,b);}

void Yes(bool f) {if(f){P("Yes");}else{P("No")}}
void yES(bool f) {if(f){P("yES");}else{P("nO")}}
void Yay(bool f) {if(f){P("Yay!");}else{P(":(")}}

void ll2str(const ll &n,string &s) { stringstream stream; stream << n; stream >> s; }
void int2str(const int &n,string &s) { stringstream stream; stream << n; stream >> s; }
void str2ll(ll &n,const string &s) { stringstream stream(s); stream >> n; }
void str2int(int &n,const string &s) { stringstream stream(s); stream >> n; }

/*********TEMPLATE**********/

ll n,d;

signed main() {

  cin >> n >> d;
  // 0:東京にいる 1:京都にいる
  ll dp[110][2];
  dp[0][0] = 0;

  reps(i,1,n+1) {
    ll t,k;
    cin >> t >> k;
    if (i==1) {
      dp[i][0] = dp[i-1][0] + t;
      dp[i][1] = dp[i-1][0] + k - d;
    } else {
      dp[i][0] = max(dp[i-1][0]+t,dp[i-1][1]+t-d);
      dp[i][1] = max(dp[i-1][0]+k-d,dp[i-1][1]+k);
    }
  }

  P(max(dp[n][0],dp[n][1]));

}