#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <iomanip>
using namespace std;
#define INF 100010
#define PI acos(-1)
typedef long long ll;
typedef pair<int, int> p_ii;



int main() {
	
	int n; cin >> n;
	vector<int> a(500);
	a[0] = 0;
	for (int i = 1; i <= n; i++)cin >> a[i];
	int sum = 0;
	for (int i = 1; i <= n; i++)sum += a[i];

	int p = 0, height;
	for (int i = 1; i < 200; i++) {
		if (p + 2 * i - 1 > sum) {
			height = i - 1;
			break;
		}
		p += 2 * i - 1;
	}

	for (int i = 1; i <= height; i++) {
		sum -= (a[i] < i) ? a[i] : i;
	}
	for (int i = height + 1; i < 2 * height; i++) {
		sum -= (a[i] < 2 * height - i) ? a[i] : 2 * height - i;
	}
	
	cout << sum << endl;

	return 0;
}