#include <bits/stdc++.h>

using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()

template<typename A, typename B> inline bool chmax(A &a, B b) { if (a<b) { a=b; return 1; } return 0; }
template<typename A, typename B> inline bool chmin(A &a, B b) { if (a>b) { a=b; return 1; } return 0; }

typedef unsigned long long ull;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const ll INF = 1ll<<29;
const ll MOD = 1000000007;
const double EPS  = 1e-10;

int main() {
	int n, a[100];
	
	cin >> n;
	REP(i, n) scanf("%d", a + i);
	
	int sum[101];
	sum[0] = 0;
	REP(i, n) sum[i + 1] = sum[i] + a[i];
	
	int cnt[10000];
	
	cnt[1] = 1;
	for (int i = 3; i < 10000; i += 2)
		cnt[i] = cnt[i - 2] + i;
	
	int maxs;
	for (int i = 1; i < 10000; i += 2)
		if (sum[n] >= cnt[i]) maxs = i;
	
	int ans = 0;
	
	if (maxs < n) ans += sum[n] - sum[maxs];
	
	if (maxs / 2 < n && a[maxs / 2] > maxs / 2 + 1) ans += a[maxs / 2] - (maxs / 2 + 1);
	
	for (int i = 0; i < maxs / 2; i++) {
		if (i >= n) break;
		
		if (a[i] > i + 1) ans += a[i] - (i + 1);
		if (maxs - i - 1 < n && a[maxs - i - 1] > i + 1) ans += a[maxs - i - 1] - (i + 1);
	}
	
	cout << ans << endl;
	
	return 0;
}