#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i = (a); i < (b); ++i)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) for(int i = (n) - 1; (i) >= 0; --i)
#define SZ(n) (int)(n).size()
#define ALL(n) (n).begin(), (n).end()
#define MOD 1000007
#define INF 100000000
using namespace std;
typedef long long LL;
typedef vector<int> VI;
typedef pair<int, int> PI;


int main() {
	int n;
	int a[202] = {};
	cin >> n;
	int sum = 0;
	REP(i, n) {
		cin >> a[i];
		sum += a[i];
	}
	int k = 1;
	int t = sum;
	while ((t -= k) >= 0) {
		k += 2;
	}
	int na[202] = {};
	t = 0;
	REP(i, (k - 1) / 2) {
		na[i] = na[k - i - 3] = i + 1;
		t += na[i];
	}
	int ans = sum;
	REP(i, k) {
		ans -= min(a[i], na[i]);
	}
	cout << ans << endl;
	return 0;
}