#include <bits/stdc++.h>

using namespace std;

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

const double PI = acos(-1);

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int n;
  cin >> n;
  vector<int> a(n+1);

  int sum = 0;
  REP (i, n) {
    cin >> a[i+1];
    sum += a[i+1];
  }

  int ret = 1e8;
  for (int i = 1; ; i++) {
    int cnt = 0;
    for (int j = 1; j <= i; j++) {
      cnt += j;
      if (j < i) cnt += j;
    }

    if (cnt > sum) break;
    int tmp = sum - cnt;
    for (int j = 1; j <= 2*i-1; j++) {
      int k = j <= i ? j : 2 * i - j;
      if (j > n || a[j] < k)
        tmp += k - a[j];
    }
    ret = min(ret, tmp);
  }

  cout << ret << endl;
  
  return 0;
}