#include<bits/stdc++.h>
using namespace std;
typedef long long ll;

#define fast ios_base::sync_with_stdio(0);cin.tie(NULL);cout.tie(NULL)

ll dp[1000099][4];
ll ans[1000099],a[1000099];
int main(){
  fast;
  ll n,c;cin>>n>>c;

  for(ll i=1;i<=n;++i){
    cin>>a[i];
  }
  if(n==1){
    cout<<0;return 0;
  }
  for(int i=2;i<=n;++i){
    for(int j=1;j<=3 && i - j >=1;++j){
        if (j == 1){
          dp[i][i-j] = max(0ll,a[i]-a[i-1]-c);
          ans[i] = max(ans[i],dp[i][i-j]+ans[i-j-1]);
        }
        if(j==2){
          dp[i][i-j] = max(0ll,max(a[i-1]-a[i-2]-c, max(a[i]-a[i-1]-c,a[i]-a[i-2]-c)));
          ans[i] = max(ans[i],dp[i][i-j]+ans[i-j-1]);
        }
        if(j==3){//i-3 i-2 i-1 i
          dp[i][i-j] = max(0ll, a[i-2] - a[i-3] - c);
          dp[i][i-j] = max(dp[i][i-j], max(a[i-1]-a[i-3]-c, a[i]-a[i-3]-c));

          dp[i][i-j]  = max(dp[i][i-j], max(a[i-1]-a[i-2]-c, a[i]-a[i-2]-c));

          dp[i][i-j]  = max(dp[i][i-j], max(a[i]+a[i-2]-a[i-3]-a[i-1]-c , a[i]-a[i-1]-c));

          ans[i] =  max(ans[i],dp[i][i-j]+ans[i-j-1]);
        }
    }
  }
  cout<<ans[n];


  return 0;
}