#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  rep(i, n) cin >> a[i];
  int ans = 1e9;
  const int start = 0;
  for (int len = 0; start + 2 * len < n; ++len) {
    int over = 0, less = 0;
    rep(i, n) {
      if (i < start || i > start + 2 * len) {
        over += a[i];
      }
      else {
        int height = (len + 1) - abs((start + len) - i);
        less += max(0, height - a[i]);
        over += max(0, a[i] - height);
      }
    }
    if (less > over) continue;
    ans = min(ans, over);
  }
  cout << ans << endl;
  return 0;
}