#include <vector>
#include <queue>
#include <deque>
#include <stack>
#include <map>
#include <set>
#include <algorithm>
#include <functional>
#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <cctype>
#include <string>
#include <cstring>

using namespace std;

typedef long long ll;


int main() {
  int n;
  cin >> n;
  int a[n];
  for (int i = 0; i < n; i++) cin >> a[i];
  int total = 0;
  for (int i = 0; i < n; i++) total += a[i];
  int ans = 1000000000;
  int cnt = 0;
  for (int i = 1; i <= n; i++) {
    if (i%2 == 0) continue;
    cnt++;
    if (total < cnt*cnt) continue;
    int sum = 0;
    for (int j = 0; j < i/2; j++) sum += max(a[j]-(j+1), 0);
    for (int j = i/2, k = i/2; j < i; j++,k--) sum += max(a[j]-(k+1), 0);
    for (int j = i; j < n; j++) sum += a[j];

    // cout << sum << " " << i << endl;
    ans = min(sum, ans);
  }

  cout << ans << endl;
}