#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
#define rep(i, n) REP(i, 0, n)
#define REP(i, s, e) for (ll i = (s); i < (ll)(e); i++)
#define repr(i, n) REPR(i, n, 0)
#define REPR(i, s, e) for (ll i = (ll)(s - 1); i >= (ll)(e); i--)
#define all(r) r.begin(), r.end()
#define rall(r) r.rbegin(), r.rend()

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;

template <typename T, typename U>
bool chmax(T& a, const U& b) {
    if (a >= b) return false;
    a = b;
    return true;
}
template <typename T, typename U>
bool chmin(T& a, const U& b) {
    if (a <= b) return false;
    a = b;
    return true;
}

void yes_no(bool f, string yes = "Yes", string no = "No") {
    cout << (f ? yes : no) << "\n";
}

vector<string> split_string(const string& s, char c = ' ') {
    vector<string> res;
    stringstream ss(s);
    string tmp;
    while (getline(ss, tmp, c)) {
        res.emplace_back(tmp);
    }
    return res;
}

void solve() {
    int n;
    cin >> n;
    vl a(n);
    rep(i, n) cin >> a[i];
    ll sum = accumulate(all(a), 0LL);

    const ll inf = 1e18;
    ll ans = inf;
    for (int i = 1; i < 200; i += 2) {
        ll x = 0, y = 0, z = 0;
        rep(j, (i + 1) / 2) {
            z++;
            if (j < n) {
                if (a[j] < z) x += z - a[j];
                else y += a[j] - z;
            } else {
                x += z;
            }
        }
        z = 0;
        for (int j = i - 1; j >= (i + 1) / 2; --j) {
            z++;
            if (j < n) {
                if (a[j] < z) {
                    x += z - a[j];
                } else {
                    y += a[j] - z;
                }
            } else {
                x += z;
            }
        }
        REP(j, i, n) {
            y += a[j];
        }
        if (y >= x) {
            chmin(ans, y);
        }
    }
    cout << ans << "\n";
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int t = 1;
    // cin >> t;
    rep(ti, t) solve();
    return 0;
}