#include "bits/stdc++.h"
using namespace std;

#define DEBUG(x) cout << #x << ": " << x << endl;
#define DEBUG_VEC(v)                                      \
  cout << #v << ":";                                      \
  for (int i = 0; i < v.size(); i++) cout << " " << v[i]; \
  cout << endl

typedef long long ll;
#define vi vector<int>
#define vl vector<ll>
#define vii vector<vector<int> >
#define vll vector<vector<ll> >
#define vs vector<string>
#define pii pair<int, int>
#define pis pair<int, string>
#define psi pair<string, int>
const int inf = 1000000001;
const ll INF = 1e16;
#define MOD 1000000007
#define mod 1000000009
#define pi 3.14159265358979323846
#define Sp(p) cout << setprecision(15) << fixed << p << endl;
int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};

int main(){
  int n,d,j,i;
  cin>>n>>d;
  vi t(n),k(n);
  for(i=0;i<n;i++){
    cin>>t[i]>>k[i];
  }
  bool basyo = true;
  ll ans = 0;
  vll dp(2,vl(n+1,0));
  dp[1][0] = -INF;
  for(j=1;j<=n;j++){
    dp[0][j] = max(dp[0][j-1] + t[j-1],dp[1][j-1] + t[j-1] - d);
    dp[1][j] = max(dp[1][j-1] + k[j-1],dp[0][j-1] + k[j-1] - d);
  }
  cout<<max(dp[0][n],dp[1][n])<<endl;
}