import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.TreeSet;
public class Main {
	public static int w;
	public static int k;
//	public static int n;
//	public static int[][] dp;
//	public static int[] r;
//	public static long[] buy;
//	public static List<double[]> list = new ArrayList<double[]>();
//	public static TreeSet<Integer> set = new TreeSet<Integer>();
//	public static PriorityQueue<Integer> pq = new PriorityQueue<Integer>();
	public static void main(String[] args) throws NumberFormatException, IOException{

		ContestScanner in = new ContestScanner();
		int n = in.nextInt();
		int count = 0;
		int[] renga = new int[n];
		int rengaNum = 0;
		for(int i=0; i<n; i++){
			renga[i] = in.nextInt();
			rengaNum += renga[i];
		}
		int base = 0;
		for(int i=1; i<=100; i++){
			if(i*i >= rengaNum){
				base = i;
				break;
			}
		}
		
		base = base*2-1;
		int max = Math.min(base, n);
		int subc = 0;
		for(int i=0; i<max; i++){
			if(i<max/2){
				int val = renga[i] - (i+1);
				if(val > 0) count += val;
			}else{
				int val = renga[i] - (n-i);
				if(val > 0)	count += val;
			}
		}
		if(base < n){
			for(int i=base; i<n; i++) count += renga[i];
		}else if(base > n){
			int[] renga2 = new int[base];
			for(int i=0; i<base; i++){
				renga2[i] = renga[i];
			}
			for(int i=0; i<base/2; i++){
				if(i<n/2){
					int val = renga2[i] - (i+1);
					if(val > 0) subc += val;
				}else{
					int val = renga2[i] - (n-i);
					if(val > 0)	subc += val;
				}

			}
				
		}
		count = Math.max(count, subc);
		
		
		
		System.out.println(count);
	}
	
}

class Range<T extends Comparable<T>> implements Comparable<Range<T>>{
	T start;
	T end;
	public Range(T start, T end){
		this.start = start;
		this.end = end;
	}
	public boolean inRange(T val){
		if(start.compareTo(val) <= 0 && end.compareTo(val) >= 0){
			return true;
		}
		return false;
	}
	
	public boolean isCommon(Range<T> range){
		if(inRange(range.start) || inRange(range.end) || range.inRange(start)){
			return true;
		}
		return false;
	}
	
	public Range<T> connect(Range<T> range){
		if(!isCommon(range)) return null;
		Range<T> res = new Range<T>(start.compareTo(range.start) <= 0 ? start : range.start,
				end.compareTo(range.end) >= 0 ? end : range.end);
		return res;
	}
	
	public boolean connectToThis(Range<T> range){
		if(!isCommon(range)) return false;
		start = start.compareTo(range.start) <= 0 ? start : range.start;
		end = end.compareTo(range.end) >= 0 ? end : range.end;
		return true;
	}
	@Override
	public int compareTo(Range<T> range) {
		int res = start.compareTo(range.start);
		if(res == 0) return end.compareTo(range.end);
		return res;
	}
	
	public String toString(){
		return "["+start+","+end+"]";
	}
}

class RangeSet<T extends Comparable<T>>{
	TreeSet<Range<T>> ranges = new TreeSet<Range<T>>();
	public void add(Range<T> range){
		Range<T> con = ranges.floor(range);
		if(con != null){
			if(con.connectToThis(range))
				range = con;
		}
		con = ranges.ceiling(range);
		while(con != null && range.connectToThis(con)){
			ranges.remove(con);
			con = ranges.ceiling(range);
		}
		ranges.add(range);
	}
	
	public String toString(){
		StringBuilder bld = new StringBuilder();
		for(Range<T> r: ranges){
			bld.append(r+"\n");
		}
		return bld.toString();
	}
}


class Node{
	int id;
	List<Node> edge = new ArrayList<Node>();
	public Node(int id){
		this.id = id;
	}
	public void createEdge(Node node){
		edge.add(node);
	}
}

class MyMath{
	public static long fact(long n){
		long res = 1;
		while(n > 0){
			res *= n--;
		}
		return res;
	}
	public static long[][] pascalT(int n){
		long[][] tri = new long[n][];
		for(int i=0; i<n; i++){
			tri[i] = new long[i+1];
			for(int j=0; j<i+1; j++){
				if(j == 0 || j == i){
					tri[i][j] = 1;
				}else{
					tri[i][j] = tri[i-1][j-1] + tri[i-1][j];
				}
			}
		}
		return tri;
	}
}

class Reverse implements Comparator<Integer>{
	public int compare(Integer arg0, Integer arg1) {
		return arg0 - arg1;
	}
}

class ContestScanner{
	private BufferedReader reader;
	private String[] line;
	private int idx;
	public ContestScanner() throws FileNotFoundException{
		reader = new BufferedReader(new InputStreamReader(System.in));
	}
	
	public String nextToken() throws IOException{
		if(line == null || line.length <= idx){
			line = reader.readLine().trim().split(" ");
			idx = 0;
		}
		return line[idx++];
	}
	
	public long nextLong() throws IOException, NumberFormatException{
		return Long.parseLong(nextToken());
	}
	
	public int nextInt() throws NumberFormatException, IOException{
		return (int)nextLong();
	}
	
	public double nextDouble() throws NumberFormatException, IOException{
		return Double.parseDouble(nextToken());
	}
}