#include<bits/stdc++.h>
using namespace std;  
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
#define rep(i,n) for(ll i=0;i<(n);i++)  
#define pii pair<int,int>
#define piii pair<int,pii>
#define mp make_pair
#define pb push_back  
#define ALL(a) (a).begin(),(a).end()
#define FST first
#define SEC second  
const int INF = (INT_MAX/2);
const ll LLINF = (LLONG_MAX/2);
const double eps = 1e-5;
const double PI = M_PI;  
#define DEB cout<<"!"<<endl
#define SHOW(a,b) cout<<(a)<<" "<<(b)<<endl
#define SHOWARRAY(ar,i,j) REP(a,i)REP(b,j)cout<<ar[a][b]<<((b==j-1)?((a==i-1)?("\n\n"):("\n")):(" "))
  
#define DIV 1000000007
typedef vector<ll> Array;
typedef vector<Array> matrix;
typedef tuple<int,int,int> tiii;
#define mt make_tuple

int n; ll c;
vector<ll> tokyo;
vector<ll> kyoto;
ll ans = 0;
ll memo[100][2] = {};
void dfs(ll sum,int day,int now){
  if(memo[day][now] > sum)  return;
  else  memo[day][now] = sum;
  if(day == n){
    ans = max(sum,ans);
  }else{
    if(now == 0){
      dfs(sum-c+kyoto[day],day+1,1);
      dfs(sum+tokyo[day],day+1,0);
    }else{
      dfs(sum-c+tokyo[day],day+1,0);
      dfs(sum+kyoto[day],day+1,1);
    }
  }
}

int main(){
  fill_n(memo[0],100*2,0);
   cin >> n >> c;
  // 0:tokyo , 1:kyoto
  rep(i,n){
    ll t,k; cin >> t >> k;
    tokyo.pb(t); kyoto.pb(k);
  }
  dfs(0,0,0);
  cout << ans << endl;
}