#include <bits/stdc++.h>

using namespace std;

const int maxn = 110000;

int a[maxn];
int e[maxn];

int d[maxn][40];

int n;

int main() {
  cin>>n;
  for (int i=0;i<n;i++) cin>>a[i];
  e[n-1] = a[n-1];
  for (int i=n-2;i>=0;i--)
    e[i] = max(a[i], e[i+1]);
  for (int i=0;i<n;i++) d[i][0]=a[i];

  for (int k=1;(1<<k) <=n; k++) {
    int half = 1<<(k-1);
    int full = 1<<k;
    for (int i=0;i<n;i++) {
      if (i+full<=n) d[i][k] = max(d[i][k-1], d[i+half][k-1]);
      else d[i][k] = e[i];
    }
  }

  bool found =0;
  int best;

  for (int s=0;s<n;s++) {
    for (int k=0;s+(1<<k)<n;k++) {
      int x = a[s] + d[s+(1<<k)][k] - k;
    
      if (!found || x>best) {
	best = x;
	found = true;
      }
    }
  }

  cout<<best<<endl;

  return 0;
}