#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[101];
	
	cnt[1] = 1;
	for (int i = 3; i <= n; i+= 2)
		cnt[i] = cnt[i - 2] + i;
	
	int ans = INF;
	
	for (int i = 1; i <= n; i += 2) {
		if (sum[n] < cnt[i]) break;
		
		int now = sum[n] - sum[i];
		
		if (a[i / 2] > i / 2 + 1) now += a[i / 2] - (i / 2 + 1);
		
		for (int j = 0; j < i / 2; j++) {
			if (a[j] > j + 1) now += a[j] - (j + 1);
			if (a[i - j - 1] > j + 1) now += a[i - j - 1] - (j + 1);
		}
		
		chmin(ans, now);
	}
	
	cout << ans << endl;
	
	return 0;
}