rt

#include<cstdio>
#include<cstring>
#include<algorithm>

using namespace std;

const int N=4e6+10;
typedef long long ll;

int n,a[N];
ll a1[N],a2[N],ans,mn;

int main(){
	
	scanf("%d",&n);
	for(int i=1;i<=n;i++) scanf("%d",&a[i]),a1[i]=a[i]-i,a2[i]=a[i]-(n-i+1);
	
	ans=-1e9; mn=1e9;
	for(int i=1;i<=n;i++){
		ans=max(ans,a1[i]-mn-1);
		mn=min(mn,a1[i]);
	}
	
	mn=1e9;
	for(int i=n;i>=1;i--){
		ans=max(ans,a2[i]-mn-1);
		mn=min(mn,a2[i]);
	}
	
	printf("%lld\n",ans);
	
	return 0;
}