#define _USE_MATH_DEFINES
#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<string>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include<queue>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include <sstream>
#include<iomanip>

//#include<bits/stdc++.h>


using namespace std;



int main() {
	int  n;
	int a = 4;
	cin >> n;
	int b = 0;
	vector<int>t;
	for (int i = 0; i < n; i++) {
		int x;
		cin >> x;
		t.push_back(x);
		b += x;
	}
	int x = 2;
	while (b >= a+2*x+1) {
		
		a += 2 * x + 1;
		x++;
	}
	int s = 0,d=0;
	int y = 0;
	
	for (int i = 0; i < min(2*x-1,n); i++) {
		if (i > x - 1)y--;
		else y++;
		if (t[i] > y)s += t[i] - y;
		d += t[i];
	//	cout << s << " "<<y << endl;
	}
	s += b - d;
	cout << s << endl;
	
	return 0;
}